Add an adaptive_sync output command

This enables/disables adaptive synchronization on the output.

For now, the default is disabled because it might cause flickering on
some hardware if clients don't submit frames at regular enough
intervals. In the future an "auto" option will only enable adaptive sync
if a fullscreen client opts-in via a Wayland protocol.

include/sway/config.h

@@ -238,6 +238,7 @@ struct output_config {
 	int32_t transform;
 	enum wl_output_subpixel subpixel;
 	int max_render_time; // In milliseconds
+	int adaptive_sync;
 
 	char *background;
 	char *background_option;