GPU_select originally used GL_SELECT which defined the format for storing the selection result. Now this is no longer the case, define our own struct - making the code easier to follow: - Avoid having to deal with arrays in both `uint*` and `uint(*)[4]` multiplying offsets by 4 in some cases & not others. - No magic numbers for the offsets of depth & selection-ID. - No need to allocate unused members to match GL_SELECT (halving the buffer size).