layout: Don't rebuild display lists at all unless restyling tells us

some object needs to be repainted.

Reduces CPU usage when mousing over simple documents. (More complex
documents tend to trigger unnecessary reflow bugs and so still have high
CPU.)

Part of #9999.

components/layout/traversal.rs

@@ -234,6 +234,6 @@ impl<'a> BuildDisplayList<'a> {
 
     #[inline]
     fn should_process(&self) -> bool {
-        self.state.layout_context.shared_context().goal == ReflowGoal::ForDisplay
+        true
     }
 }