Not sure why but adding a handler to the "expose-event" signal of
GimpDisplayShell (similarly to how we do it in master branch on "draw")
just didn't work. But it works on the already existing signal handling
on the canvas instead (which actually is not a bad deal, as we also
remove the coordinates translation so maybe we should test this on
`master` too).
Note: why we are backporting all this logics to gimp-2-10 is because
changes in macOS BigSur broke the selection's marching ants the same way
they broke on Wayland and it was confirmed this fix worked for BigSur as
well, at least on the dev builds.
It is unnecessary to backport for Wayland (because GIMP 2.10 is based on
GTK2 which anyway works only through XWayland, hence doesn't have the
issue), we do it only for macOS BigSur (and further). Well at least the
fix will hopefully work on the stable branch, because I cannot test
myself.
See issue #5952.
6be014fc59