pan/mdg: Implement texture gathers

pan/mdg: Implement texture gathers

As previously shown, it is a mode on top of textureLod. The main gotcha
is the results are swizzled; we reuse the Broadcom lowering for that.

Also, there's a pretty significant erratum affecting gathers of cubemaps
which can be dealt with... eventually.

Fixes:

   dEQP-GLES31.functional.texture.gather.basic.2d.*
   dEQP-GLES31.functional.texture.gather.basic.2d_array.*

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6516>