I noticed my system would fall back to the slow unclipped (and
uncullable) paint path whenever a window touched the left edge of
the screen. Turns out that was a red herring. Just that
`use_clipped_redraw` was uninitialized so clipping/culling was used
randomly.
So the compiler failed to notice `use_clipped_redraw` was uninitialized.
Weirdly, as soon as you fix that it starts complaining that `buffer_age`
might be uninitialized, which appears to be wrong. So we initialize that
too, to shut up the compiler warnings/errors.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1323
f511f94aa2