libweston: remove previous_damage from struct weston_output

The member previous_damage from struct weston_output is no longer necessary.
First, stop calling init, fini and copying output_damage to it. Then remove
it from struct weston_output.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>

include/libweston/libweston.h

@@ -249,8 +249,6 @@ struct weston_output {
 	/** Output area in global coordinates, simple rect */
 	pixman_region32_t region;
 
-	pixman_region32_t previous_damage;
-
 	/** True if damage has occurred since the last repaint for this output;
 	 *  if set, a repaint will eventually occur. */
 	bool repaint_needed;