wlr_scene: Debounce dmabuf feedback on scanout

Direct scanout can be enabled and disabled on a frame-by-frame basis,
and so we could end up sending different feedback to a surface on every
other frame. Reacting to new feedback is expensive, as the client may
need to reallocate their swapchain.

Debounce the state change a number of frames, for now set to 30, to
avoid immediate reaction to scanout (or composition) that only lasts a
few frames.

A timer could be used instead, but it did not seem worth the complexity.
What just want to know that the state has been stable across a
reasonable number of samples, and a counter seems sufficient for that.

include/wlr/types/wlr_scene.h

@@ -227,6 +227,14 @@ struct wlr_scene_output {
 		pixman_region32_t pending_commit_damage;
 
 		uint8_t index;
+
+		/**
+		 * When scanout is applicable, we increment this every time a frame is rendered until
+		 * DMABUF_FEEDBACK_DEBOUNCE_FRAMES is hit to debounce the scanout dmabuf feedback. Likewise,
+		 * when scanout is no longer applicable, we decrement this until zero is hit to debounce
+		 * composition dmabuf feedback.
+		 */
+		uint8_t dmabuf_feedback_debounce;
 		bool prev_scanout;
 
 		bool gamma_lut_changed;