pan/mdg: add support for 2DMS/2DMSArray images

Tested with the following tests on desktop GL 4.4:

KHR-GL4[0-4].shader_image_size.advanced-ms-{cs,fs,vs}-{float,int,uint}
KHR-GL4[0-4].shader_subroutine.subroutine_uniforms_used_for_sampling_atomic_image_functions
KHR-GL4[0-4].copy_image.samples_mismatch
KHR-GL4[0-4].shader_image_load_store.advanced-sso-perSample
KHR-GL4[0-4].shader_image_load_store.basic-allTargets-load-ms

At least 48 new tests passing.

cc @bbrezillon @alyssa

Not sure if you'll want to merge this, but here it is.