If full output damage is forced every time a screenshot is taken, the
test suite cannot take screenshots of damage as that would depend on
repainting only the damaged area.

Stop damaging the output and instead just schedule a repaint so the
screenshot can be taken. This is safe because:

- if any views were promoted to hw planes previously,
  weston_output_disable_planes_incr() would force them to be demoted at
  assing_planes() time causing damage via weston_view_move_to_plane()

- even when hardware primary plane has no damage, DRM-backend will not
  skip calling to the renderer after commit
  "drm-backend: do not skip renderer if capturing screen".

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>