This will let us use a subset of the full texture, which can
be necessary in the case that converters put padding around
content in dmabufs. The naming follows the Wayland viewporter
spec.
For now, make all callers pass the full texture rect.
We are going to introduce another rect, so better to be clear in
naming. We are following the naming of the Wayland viewporter spec
and call the rectangle that we drawing into the dest(ination).
Instead of relying on diffing subsurface nodes, we track damage
generated by offloaded contents inside GskOffload.
There are 3 stages a subsurface node can be in:
1. not offloaded
Drawing is done by the renderer
2. offloaded above
The renderer draws nothing
3. offloaded below
The renderer needs to punch a hole.
Whenever the stage changes, we need to repaint.
And that can happen without the subsurface's contents changing, like
when a widget is put above the subsurface and it needs to to go from
offloaded above to below.
So we now recruit GskOffload for tracking these changes, instead of
relying on the subsurface diffing.
But we still need the subsurface diffing code to work for the
non-offloaded case, because then the offloading code is not used.
So we keep using it whenever that happens.
Not that when a subsurface transitions between being offloaded and not
being offloaded, we may diff it twice - once in the offload code and
once in the node diffing - but that shouldn't matter.
