For backend handled cursors, if nothing else changes on the clutter
stage, we end up not sending out frame callbacks since clutter doesn't
draw a new frame.
To fix this, we'll keep cursor surfaces' frame callbacks separate from
other surfaces' and trigger them from the new
MetaCursorRenderer::cursor-painted signal which handles both software
and hardware cursors.
https://bugzilla.gnome.org/show_bug.cgi?id=749913
Move into a standalone meta-wayland-surface-role-cursor.[ch], and
make generic enough to work for pointe and additional (eg. tablet)
cursors.
Most notably, the sprite is now kept completely internal to the
cursor role, and updates are routed through the given
MetaCursorRenderer (which may be the default one for the pointer,
or something else).
The way cursor updates after cursor surface destruction has also
been reworked, the pointer will just keep track of the last cursor
surface, so older surfaces being destroyed don't trigger pointer
rechecks/updates.
