Track latest VTM version using CMake's FetchContent

[frankplow]

This PR uses CMake's FetchContent to automatically fetch the latest version of VTM from the official repo rather than redistributing it. This should reduce the workload maintaining this repo. Draft for now as the updates to VTM between 9.3 and 20.0 have broken the library and it currently fails to build.

[frankplow]

Here are the build errors at the moment:

[ 38%] Building CXX object CMakeFiles/vtmdecoder.dir/src/libVTMDecoder.cpp.o
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:201:51: error: no member named 'getNumReorderPics' in 'SPS'; did you mean 'getMaxNumReorderPics'?
      this->numReorderPicsHighestTid = activeSPS->getNumReorderPics( temporalId );
                                                  ^~~~~~~~~~~~~~~~~
                                                  getMaxNumReorderPics
/Users/frank/Downloads/VTM/build/_deps/vtm-src/source/Lib/CommonLib/./SequenceParameterSet.h:718:27: note: 'getMaxNumReorderPics' declared here
  int                     getMaxNumReorderPics(uint32_t tlayer) const            { return m_maxNumReorderPics[tlayer]; }
                          ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:206:53: error: no member named 'getNumReorderPics' in 'VPS'; did you mean 'getMaxNumReorderPics'?
      this->numReorderPicsHighestTid = referredVPS->getNumReorderPics( temporalId );
                                                    ^~~~~~~~~~~~~~~~~
                                                    getMaxNumReorderPics
/Users/frank/Downloads/VTM/build/_deps/vtm-src/source/Lib/CommonLib/./VideoParameterSet.h:213:21: note: 'getMaxNumReorderPics' declared here
  int               getMaxNumReorderPics( int temporalId ) const         { return m_dpbParameters[m_olsDpbParamsIdx[m_targetOlsIdx]].maxNumReorderPics[temporalId]; }
                    ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:284:10: error: use of undeclared identifier 'NAL_UNIT_RESERVED_IRAP_VCL_12'; did you mean 'NAL_UNIT_RESERVED_IRAP_VCL_11'?
    case NAL_UNIT_RESERVED_IRAP_VCL_12:
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         NAL_UNIT_RESERVED_IRAP_VCL_11
/Users/frank/Downloads/VTM/build/_deps/vtm-src/source/Lib/CommonLib/./TypeDef.h:929:3: note: 'NAL_UNIT_RESERVED_IRAP_VCL_11' declared here
  NAL_UNIT_RESERVED_IRAP_VCL_11,
  ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:284:10: error: duplicate case value 'NAL_UNIT_RESERVED_IRAP_VCL_11'
    case NAL_UNIT_RESERVED_IRAP_VCL_12:
         ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:283:10: note: previous case defined here
    case NAL_UNIT_RESERVED_IRAP_VCL_11:
         ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:482:103: error: use of undeclared identifier 'NAL_UNIT_RESERVED_IRAP_VCL_12'; did you mean 'NAL_UNIT_RESERVED_IRAP_VCL_11'?
      if (!bNewPicture && ((nalu.m_nalUnitType >= NAL_UNIT_CODED_SLICE_TRAIL && nalu.m_nalUnitType <= NAL_UNIT_RESERVED_IRAP_VCL_12)
                                                                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                                                                      NAL_UNIT_RESERVED_IRAP_VCL_11
/Users/frank/Downloads/VTM/build/_deps/vtm-src/source/Lib/CommonLib/./TypeDef.h:929:3: note: 'NAL_UNIT_RESERVED_IRAP_VCL_11' declared here
  NAL_UNIT_RESERVED_IRAP_VCL_11,
  ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:647:55: error: use of undeclared identifier 'CHROMA_444'
    if (c == LIBVTMDEC_LUMA || pcPic->chromaFormat == CHROMA_444)
                                                      ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:649:32: error: use of undeclared identifier 'CHROMA_422'
    if (pcPic->chromaFormat == CHROMA_422 || pcPic->chromaFormat == CHROMA_420)
                               ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:649:69: error: use of undeclared identifier 'CHROMA_420'
    if (pcPic->chromaFormat == CHROMA_422 || pcPic->chromaFormat == CHROMA_420)
                                                                    ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:662:55: error: use of undeclared identifier 'CHROMA_444'
    if (c == LIBVTMDEC_LUMA || pcPic->chromaFormat == CHROMA_444 || pcPic->chromaFormat == CHROMA_422)
                                                      ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:662:92: error: use of undeclared identifier 'CHROMA_422'
    if (c == LIBVTMDEC_LUMA || pcPic->chromaFormat == CHROMA_444 || pcPic->chromaFormat == CHROMA_422)
                                                                                           ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:664:32: error: use of undeclared identifier 'CHROMA_420'
    if (pcPic->chromaFormat == CHROMA_420)
                               ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:721:32: error: use of undeclared identifier 'CHROMA_400'
    if (pcPic->chromaFormat == CHROMA_400)
                               ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:723:32: error: use of undeclared identifier 'CHROMA_420'
    if (pcPic->chromaFormat == CHROMA_420)
                               ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:725:32: error: use of undeclared identifier 'CHROMA_422'
    if (pcPic->chromaFormat == CHROMA_422)
                               ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:727:32: error: use of undeclared identifier 'CHROMA_444'
    if (pcPic->chromaFormat == CHROMA_444)
                               ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:742:24: error: no member named 'recon' in 'EnumArray<int, ChannelType>'
      return bitDepths.recon[CHANNEL_TYPE_LUMA];
             ~~~~~~~~~ ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:742:30: error: use of undeclared identifier 'CHANNEL_TYPE_LUMA'
      return bitDepths.recon[CHANNEL_TYPE_LUMA];
                             ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:744:24: error: no member named 'recon' in 'EnumArray<int, ChannelType>'
      return bitDepths.recon[CHANNEL_TYPE_CHROMA];
             ~~~~~~~~~ ^
/Users/frank/Downloads/VTM/src/libVTMDecoder.cpp:744:30: error: use of undeclared identifier 'CHANNEL_TYPE_CHROMA'
      return bitDepths.recon[CHANNEL_TYPE_CHROMA];

[frankplow]

6881f5a updates libVTMDecoder to work with the newest VTM, version 20.0, fixing the build errors above. Seems to work as expected for me so far.