Make scan and histogram shaders compatible

[pbbastian]

Options:

1. Store histogram as AOS and make version of scan that operates on this structure.
2. Store histogram as SOA and run a scan for each array using an offset per dispatch.

Option 1 has some advantages for smaller input sizes, as it can do all the scans at once. OTOH it increases register usage which is bad for performance. Larger input sizes should satisfy option 2 just fine, which could then be made to operate on multiple inputs from the same sequence, thus not increasing register usage as much.

[pbbastian]

Option 2 is chosen.

• Make scan shader use an offset and length
• Make global scan shader use an offset and length
• Make group add shader use an offset and length
• Make histogram shader output a structure of arrays

[pbbastian]

Implemented and tested