d3d12: Reuse sampler tables inside of a batch

If a shader's sampler state is dirty often, the sampler descriptor heap can get used up quickly, forcing flushing. If that happens quickly, we run out of batches and have to wait for batches to finish on the GPU.

When this happens, it is often because the sampler state is switching, not because it's truly unique. This change hashes and saves sampler descriptor tables that can be reused in subsequent draws in the same batch, instead of re-copying the same descriptors and consuming the heap.