compositor: add screenshooter destructor

Nothing was freeing the allocation from screenshooter_create().

Add enough boilerplate, that we can free it. Fixes a Valgrind leak.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/compositor/compositor.c b/compositor/compositor.c
index d710d5f..4800971 100644
--- a/compositor/compositor.c
+++ b/compositor/compositor.c
@@ -2006,7 +2006,7 @@
 	ec->fade.animation.frame = fade_frame;
 	wl_list_init(&ec->fade.animation.link);
 
-	screenshooter_create(ec);
+	ec->screenshooter = screenshooter_create(ec);
 
 	wlsc_data_device_manager_init(ec);
 
@@ -2036,6 +2036,9 @@
 
 	wl_event_source_remove(ec->idle_source);
 
+	if (ec->screenshooter)
+		screenshooter_destroy(ec->screenshooter);
+
 	/* Destroy all outputs associated with this compositor */
 	wl_list_for_each_safe(output, next, &ec->output_list, link)
 		output->destroy(output);