Fix source crop in the pixman render backend. It was being applied by
using a source offset as arguments to pixman_image_composite32(). But
this is wrong because the source crop should get applied before all the
other transforms, not after them (or at least this is how it works in
the other wlroots render backends). Instead, apply the source crop as
yet another matrix transform when we're doing transforms (Or keep it the
same as previously if there's no other transforming going on).
Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3830
The translations for flipped offsets were set the same as non-flipped
ones which was totally wrong and meant that any textures with
flipped-transforms rendered entirely outside the viewport and were
basically invisible.
David Turner
Alexander Orzechowski
Simon Ser