vkd3d: Allow overriding the device capabilities.

This goes atop !981 (merged) and indirectly obsoletes !973 (closed).

Since we were overestimating them, this patch will likely result into reducing the reported resource binding tier in many configurations, which could in turn make some game refuse to run. I have no numbers here, but we might consider introducing an environment variable to fake the reported tier (in principle also for other variables, like the feature level). Might that be a good idea?