st/mesa: simplify and fix st_GetTexSubImage

st/mesa: simplify and fix st_GetTexSubImage

By using _mesa_image_address, the code becomes simpler _and_ fixes the bug
that GL_PACK_SKIP_IMAGES was applied even on non-3D textures.

Also, converting a whole slice at a time simplifies the format translation
fallback path.

Fixes parts of GL45-CTS.gtf32.GL3Tests.packed_pixels.packed_pixels_pixelstore.

v2: fix a silly mistake during code movement

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>