zink: emulate optional depth-formats

zink: emulate optional depth-formats

The Vulkan spec says that an implementation has to support one of
VK_FORMAT_X8_D24_UNORM_PACK32 and VK_FORMAT_D32_SFLOAT, as well of
one of VK_FORMAT_D24_UNORM_S8_UINT and VK_FORMAT_D32_SFLOAT_S8_UINT.

So let's keep track which one is supported of earch pair, and emulate
one on top of the other one.

This won't give the exact result for comparisons, or when mapping and
unmapping the resources. But it's better than flat out failing to create
the resource, and we can fix the map/unmap issue later if needed.

Tested-by: Duncan Hopkins <duncan@thefoundry.co.uk>