[Rend2] Fix Ghoul2 models being rendered twice
[Rend2] Fix out of bounds access

The normal map generation code was accessing out of array bounds. This
commit fixes the problem.
[Rend2] glsl_shaders.cpp is now autogenerated.

Added a new target to the CMake files to pack all of the .glsl files in
the glsl/ folder into a single .cpp file. This will be automatically run
every time the rend2 library is built, and if any of the .glsl files have
changed.
[Rend2] Updated GLSL files for skeletal animation.