compositor: repaint backends separately

When a repaint is triggered then not all backends may have outputs that need to
be repainted.
Check which outputs will be repainted first and then repaint only the backends
that need it.
This way unnecessary repaint_begin() and repaint_flush()/repaint_cancel() can
be skipped and errors are handled for each backend separately.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>