Support layer: Force coherent memory

[solidpixel]

Some content using a cached-but-non-coherent is missing the required coherency operations so accesses stale data. We should add an option to swap all of these allocations to a cached-coherent heap (if available), or a not-cached-coherent heap (which is guaranteed to be available).

Worth noting that this may have issues on discrete GPUs, where heap sizes are not uniform, but I plan to ignore this as it isn't an issue for mobile with unified memory.