Bug 1595014 - Make dirty rects are invalid when old tiles were droppe…

…d r=gw

composite_simple() calculates combined dirty rect from all tile's dirty rect. But the combined dirty rect becomes invalid when there is an old tile that was dropped. The dropped tile's dirty rect is not counted in composite_simple().

Differential Revision: https://phabricator.services.mozilla.com/D62531

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/8e08016b3d0acf0c5c3116b087d5455df573d99c

webrender/src/picture.rs

@@ -2428,6 +2428,11 @@ impl TileCacheInstance {
             }
         }
 
+        // When old tiles that remain after the loop, dirty rects are not valid.
+        if !self.old_tiles.is_empty() {
+            frame_state.composite_state.dirty_rects_are_valid = false;
+        }
+
         // Any old tiles that remain after the loop above are going to be dropped. For
         // simple composite mode, the texture cache handle will expire and be collected
         // by the texture cache. For native compositor mode, we need to explicitly