Disentangle frame callback sequence from real DRM sequence

It can currently happen that the estimated VBlank timer fires right
before a real VBlank, which can cause some sequence collisions, which
might cause frame callbacks to never be sent. To prevent this, just
track the frame callback sequence fully separately. There isn't really
any harm in this, and if we accidentally increment it more frequently
than necessary then nothing terrible will happen.

src/backend/winit.rs

@@ -199,6 +199,10 @@ impl Winit {
             RedrawState::WaitingForEstimatedVBlankAndQueued(_) => unreachable!(),
         }
 
+        output_state.frame_callback_sequence = output_state.frame_callback_sequence.wrapping_add(1);
+
+        // FIXME: this should wait until a frame callback from the host compositor, but it redraws
+        // right away instead.
         if output_state.unfinished_animations_remain {
             self.backend.window().request_redraw();
         }