This is the conventional way of dealing with unused arguments in C++,
since it works on all compilers.
Regex find and replace: `UNUSED\((\w+)\)` -> `/*$1*/`
The only real difference between `GPU_SHADER_2D_UNIFORM_COLOR` and
`GPU_SHADER_3D_UNIFORM_COLOR` is that in the vertex shader the 2D
version uses `vec4(pos, 0.0, 1.0)` and the 3D version uses
`vec4(pos, 1.0)`.
But VBOs with 2D attributes work perfectly in shaders that use 3D
attributes. Components not specified are filled with components from
`vec4(0.0, 0.0, 0.0, 1.0)`.
So there is no real benefit to having two different shader versions.
This will simplify porting shaders to python as it will not be
necessary to use a 3D and a 2D version of the shaders.
In python the new name for '2D_UNIFORM_COLOR'' and '3D_UNIFORM_COLOR'
is 'UNIFORM_COLOR', but the old names still work for backward
compatibility.
Differential Revision: https://developer.blender.org/D15836
