[libplacebo] Add libplacebo port

[nikitalita]

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Depends on #48995

Adds a port for libplacebo.

I am not sure which default features make sense for this; the build files in upstream state:

if not (components.get('vulkan') or components.get('opengl') or components.get('d3d11'))
  warning('Building without any graphics API. libplacebo built this way still ' +
          'has some limited use (e.g. generating GLSL shaders), but most of ' +
          'its functionality will be missing or impaired!')
endif

has_spirv = components.get('shaderc') or components.get('glslang')
needs_spirv = components.get('vulkan') or components.get('d3d11')
if needs_spirv and not has_spirv
  warning('Building without any GLSL compiler (shaderc, glslang), but with ' +
          'APIs required that require one (vulkan, d3d11). This build is very ' +
          'likely to be very limited in functionality!')
endif

So we need at least one renderer backend for this port to be any use at all, and the shader compiler dependencies are required for the renderer backends. I would appreciate advice on this.

[nikitalita]

How do you download the build logs from the CI?

[dg0yt]

The link to the instruction is in the error message.

[nikitalita]

Depends on #48995

[BillyONeal]

I am not sure which default features make sense for this;

In general due to https://learn.microsoft.com/vcpkg/contributing/maintainer-guide#default-features-should-enable-behaviors-not-apis most of the time there should be no default features.

[BillyONeal]

Drafted due to legitimate build failures.