pan/mdg: Do per-sample framebuffer loads

EXT_shader_framebuffer_fetch requires the fetched value to be per-sample, so we
need to load the sample id when in a fragment shader.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5930>
3 files changed