The rectangles need to touch/overlap in both directions, otherwise
there's no coverage that covers both rectangles.
Test included.
Fixes rendering glitches in various apps when redrawing.
Fixes: #6849
Due to the way the intermediate offscreen gets drawn, we might end up
with seams at the edges.
And I don't think it's worth spending more time on than saying "not
opaque".
Fixes the compare-render testsuite
New testcase included.
We want to be able to express opaque grids. This means that the app
provides either a row of columns of opaque nodes or a column of rows,
and then the containers will magically figure it out.
The main use case for this is terminals, which are uilt using cells. And
when there's a transparent background configured but the contents are
opaque, it'd be nice if we could figure that out.
Also remove the 80% requirement. It is rather arbitrary and while it
helps for some cases, the aforementioned grid would suffer.
Tests are node files dumped into testsuite/gsk/opaque
They are named "name-X-Y-W-H.node" with X Y W H being the expected
opaque rectangle or "name.node" if there is no opacity.
A simple example is included here.
