Texture mipmap generation doesn't work correctly on Vulkan

[frenzibyte]

Since #5508 was merged, mipmap generation has regressed on Vulkan, and the likely culprit for breaking it here is the hacky use of CopyTexture on the rendering command list, as noted by the validation layer:

I'm not sure whether this can be salvaged as-is, but we should consider moving all of the mipmap generation logic to execute on a separate command list.