script: Generate only a single frame during "update the rendering" (#38858)

Instead of generating a frame for every display list, which might be one
rendered frame per `<iframe>`, generate only a single frame per call to
"update the rendering." This should make rendering more efficient when
there are `<iframe>`s present and also open up optimizations for
non-display list frames.

Testing: This could potentially reduce flashing of content during
rendering
updates, but that is very difficult to test.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/shared/layout/lib.rs

@@ -441,6 +441,15 @@ bitflags! {
         const BuiltStackingContextTree = 1 << 2;
         const BuiltDisplayList = 1 << 3;
         const UpdatedScrollNodeOffset = 1 << 4;
+        const UpdatedCanvasContents = 1 << 5;
+    }
+}
+
+impl ReflowPhasesRun {
+    pub fn needs_frame(&self) -> bool {
+        self.intersects(
+            Self::BuiltDisplayList | Self::UpdatedScrollNodeOffset | Self::UpdatedCanvasContents,
+        )
     }
 }