Refactored duplicate Cg/GLSL LUT texture loading code into shader_common.c.
Fixed a bug where "Apply Shader Changes" in RGUI saved/set the wrong
texture filter for LUT textures.
Unofficially extended Cg shader spec with an LUT mipmap option:
To mipmap LUT "foo," add 'foo_mipmap = "true"' to your .cgp/.glslp file.
Mipmapping is currently implemented for the OpenGL driver only.
Merge branch 'improve_texturing':
OpenGL mipmap support for LUT textures is ready for upstream inclusion.