subsurface: don't forget to repaint after the first sub-surface commit
This was fixed in a very similar way in the past in eb3cf220 but was lost again in f8486c33.
As described in the original fix, if a client does this:
- create a main window and map it
- create a wl_surface, and make it a sub-surface of the main window
- set the sub-surface to desync
- commit content to the sub-surface to map it
Then step 4 should cause the sub-surface to become mapped. However, Weston fails to schedule a repaint in that case, so the sub-surface will not appear until something else causes a repaint on that output, e.g. the main window.
To avoid this, inherit the output_mask from the main surface here. With this, the commit will trigger a repaint for all outputs where the main surface is visible. This repaint will then call weston_view_update_transform() and update the output_mask correctly.