backend-wayland: don't wait for one frame when starting the repaint loop

The repaint loop is started when a client provides a new frame while the
compositor is idle. This frame should be shown as soon as possible. So it makes
no sense to commit the previous frame one more time before rendering the next
frame.

Just call weston_output_finish_frame() directly to start repainting immediately.

As a side effect, this fixes an issue when the output is resized when no
fullscreen shell is involved: At that point, parent.draw_initial_frame is
already false, so draw_initial_frame() is not called. But when resizing, the old
buffers are removed so the commit happens without a buffer attached to the
surface. So the surface is invisible for one frame until the next one is
rendered.

draw_initial_frame() is not removed here, because it is still needed for the
synchronous resize that happens with the fullscreen shell.

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