Compose the format swizzle with the descriptor swizzle (fixes sway coloring)

Gallium has two swizzles we need to respect: the format swizzle and the texture descriptor swizzle. Previously, we ignored the former, causing channel mismatches on BGR textures, for instance. This patch composes the two swizzles and uses that for the hardware instead.

This patch fixes colours in sway (previously, the red and blue channels were swapped).