[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] Added tr_worldeffect.* files to CMakeLists
Merge branch 'rend2' into weather
[Rend2] Few fixes to rain