Due to rounding errors, it is possible after intersecting a lot of
rectangles to end up with a tiny size for an offscreen. And because we
allow an epsilon before ceil()ing to an integer (see commit afc7b46264
for details) it is now possible that we end up with a size of 0.
Avoid that by always enforcing a minimum size of 1px.
Test included
The test uses a different codepath to arrive at the same problem - it
specifies the small size instead of triggering it via rounding errors
and clipping like the original bug (and most likely the more common case
to encounter this problem.
Fixes#6656
719021e1f4