[WIP] Improved transparency with dual depth peeling

[publixsubfan]

• Preliminary refactoring of rendering pipeline into stages, allowing for composable rendering operations
• Implements order-independent transparency using dual depth peeling

TODOs

• Add control for turning depth peeling on/off
• Investigate breakage on remote VPN connection
• MSAA broken with depth peeling enabled - will be addressed in another PR

[publixsubfan]

Some pictures:

[tzanio]

I could be doing something wrong, but I get the following build error on my new Mac:

g++ -O3 -std=c++11 -I../mfem-clean -DGLVIS_MULTISAMPLE=4 -DGLVIS_MS_LINEWIDTH=1.0 -DGLVIS_OGL3 -I/opt/X11/include/freetype2 ... -lfreetype -lfontconfig -lSDL2 -lGLEW -framework OpenGL -framework Cocoa -lpng -lpthread
Undefined symbols for architecture x86_64:
  "vtable for gl3::DepthPeeler", referenced from:
      gl3::DepthPeeler::DepthPeeler() in libglvis.a(aux_vis.o)
      gl3::DepthPeeler::~DepthPeeler() in libglvis.a(aux_vis.o)
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [glvis] Error 1