[WIP] Multithreaded refactoring

[publixsubfan]

Summary

Refactoring of GLVis to make it more amenable to opening multiple solutions within a single process. This avoid the need to fork in client-server mode, which may cause some issues with the native window framework on Mac OS X.

Related to #141.

Completed

• Consolidates global state in aux_vis.cpp into GLVisWindow type, which handles creation of associated VisualizationScene and GLVisCommand objects
• Move StreamReader functions into the object itself
• Modularizes palette code into PaletteState object
• Replaces pthreads in GLVisCommand with C++ standard library threads
• Consolidate some shared code between threads.cpp, glvis.cpp, aux_js.cpp
  • See StreamState::NewMeshAndSolution, StreamState::CreateVisualizationScene

To be completed

• Make VisualizationScene key event handlers rely on specific instance of GLVisWindow/VisualizationScene instead of a global pointer
• Modify SdlWindow to support multiple windows with single event loop
• Handle GLVisFont as a singleton, to avoid multiple initializations of Freetype
• Replace fork() in client-server mode with opening a new GLVisWindow on a new thread

[publixsubfan]

Closing in favor of #170 and #174.