Extend store_combined_output_pan to take a dual source blend input in
addition to colour, depth, and stencil inputs. Use the last source for
this, and represent the type with the DEST_TYPE index. This is a hack
but there is no SRC2_TYPE and NIR doesn't seem to mind as long as we
know what we mean. This allows the backend to emit a combined "blend
render target #0" instruction taking two sources.
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13714>