turnip: Use start offset to decrease alignment requirements

It turns out that there's an extra field to encode an offset in texels to apply to texel buffers, which we can use to provide the stricter alignment requirements that D3D requires. At least vkd3d was falling back to texel buffers + a separate offset buffer because we didn't support this. Now we support texel alignment for texel buffers and 4-byte alignment for storage buffers, which should be good enough for D3D. This removes offset buffers and lets vkd3d use SSBOs instead of texel buffers for raw UAVs, which should allow better vectorization.

The series is based on !19849 (merged) so as to avoid conflicting changes although it's not strictly related.