rsx: Minor fixups

[kd-11]

• Remove block alignment on compressed textures as long as the unpack alignment is well defined.
• Improve the vulkan sampler cache to not cause slowdown in games that have random parameters set. In some cases, uninitialized variables seem to make their way into unused variables but since we cannot assume they're unused, it causes a lot of permutations and can kill performance.
• Round lod bias to the nearest half integer for vulkan to work around issue 2 above when point mipmap sampling is used. Should provide near-correct results at all times while filtering out parameter noise.
• Fix incorrect variable access.