Have the OpenGL 2.1 renderer output warnings in addition to errors.

This is untested on AMD.
Fix some shaders for the new rendering system.
A big commit for getting the engine just barely running on Intel HD 3000 GPUs.

There are still a few issues remaining here:
 - Some shaders are still using GLSL 1.3+ features
 - ARB_texture_rg support is assumed
 - ARB_texture_float support is assumed

This is work towards addressing issue #60.