The only real difference between `GPU_SHADER_2D_IMAGE` and
`GPU_SHADER_3D_IMAGE` 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_IMAGE' and '3D_IMAGE'
is 'IMAGE', but the old names still work for backward
compatibility.
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