Split the function into two:
- weston_wm_window_draw_decoration() that only draws the decorations
with Cairo, and
- weston_wm_window_set_pending_state() which sets up the surface state
to be latches into use on the next commit from Xwayland.

The new weston_wm_window_do_repaint() is the equivalent of the old
weston_wm_window_draw_decorations(), everything still happens the same
way as it was. Just some debug messages have been reworded.

weston_wm_window_read_properties() is moved into
weston_wm_window_do_repaint() because it is not strictly a part of
drawing decorations. The same with resetting repaint_source.

draw_decorations does not need the child position nor xwayland
interface. Also some convenience variables have been eliminated.

set_pending_state code has been un-indented by one level, so the change
is best viewed with whitespace changes ignored.

This patch makes the code more readable, and prepares for calling the
draw_decorations and set_pending_state from different places.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>