subsurface: don't forget to repaint after the first sub-surface commit

For the following sequence, weston will not trigger a repaint:

1. create the main surface
2. create another surface and attach it as a sub-surface to the main surface
3. set the sub-surface to desync
4. attach a buffer to the main surface and commit it
5. attach a buffer to the sub-surface and commit it

Step 5 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.

And sub-surfaces are special when it comes to mapping because
weston_surface_is_mapped() will not return true until the parent surface is
mapped as well. So right now, weston_surface_map() may be called multiple times
and it will send the map_signal each time.

So to fix all this and make it clearer:

1. define a separate weston_surface_start_mapping() function to make it clearer
   that the (sub-)surface may not b...