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).
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