Fix SNAPPER_SAMPLER macros

Make sure the macros do what the comments claim that they do:
propName##Info should be a uniform.

Change-Id: I7d5d841da4d27189cd50639dbfe7bbba78dbf431
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>