Look for nodes like subsurface { clip { texture {} } }, and use
the clip to provide a source rectangle for subsetting the texture.
Update affected tests, and add a new one.
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).
We can just check if the subsurfaces contain content - and if they do,
they will be offloading and we can ignore the diff.
This essentially reverts 48740de71a
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.
When passing a directory via G_TEST_SRCDIR, still pay attention
to --verbose, and print out each file thats tests. This lets us
quickly pin down which test fails.
These tests come in two variants.
The first takes .node and .offload file, parses the node file,
and compares the resulting subsurface attachments to expected results.
The second variant takes two .node/.offload file pairs and a .diff
file, parses the node files, compares the resulting subsurface
attachments, and then diffs the nodes, comparing the resulting
area to the region in the .diff file.
