The shaders are: `GPU_SHADER_3D_FLAT_SELECT_ID` and `GPU_SHADER_3D_UNIFORM_SELECT_ID`.
This commit allows the drawing of the mesh select ids to be done on a 32UI format texture.
This simplifies the shader that previously acted on the backbuffer and had to do an uint to rgba conversion.
Differential Revision: https://developer.blender.org/D4350