We move some coding packing the texture/sampler descriptors into
dedicated functions (out of the terrifyingly long emit_for_draw
monolith), cleaning them up as we go.

The discovery triggering the cleanup is the format for including manual
strides in the presence of mipmaps/cubemaps. Rather than placed at the
end like previously assumed, they are interleaved after each address.
This difference is relevant when handling NPOT linear mipmaps.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>