Add test for rectangle path optimization with non rectilinear pen

This fails on pdf/ps.
pdf: fix rectangle stroke with non rectilinear pen
Add test for paint with alpha and clipping bug

Add test case for https://bugs.freedesktop.org/show_bug.cgi?id=68382

Something has regressed in the recording surface. All the recording
surface based backends lose the alpha from the paint_With_alpha.