GLSL loader fixes for Intel graphics

- Updated GLSL loader to use separate files for vertex and fragment
programs.
- Added a GLSL JSON format for linking vertex and fragment
programs.

This change should resolve issues loading shaders on certain Linux
systems with Intel graphics.

Fixes for two crash bugs.

- Fixes sound-related regression causing a crash inside stop_foley_sound.
- Fix for out-of-bounds thing id in level 16 end cog.