Ignore tiny error in damage when aggregating surfaces

Transforming a surface's damage into its target space involves a bit of
floating point math. This can introduce small errors - for example,
scaling 1680x945 by .8 results in 1344.000122x756.000061. If we don't
ignore this error, the inflated enclosing rectangle can prevent overlay
optimizations from operating effectively.

Bug: 1113146
Change-Id: Ic08dd08180b054b2637e57ab68b1577214bec4d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2338277Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: David Stevens <stevensd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795304}

components/viz/service/display/surface_aggregator.cc

@@ -1444,10 +1444,14 @@ gfx::Rect SurfaceAggregator::PrewalkRenderPass(
     } else {
       continue;
     }
-    // Convert the quad damage rect into its target space and clip it
-    // if needed.
-    gfx::Rect rect_in_target_space = cc::MathUtil::MapEnclosingClippedRect(
-        quad->shared_quad_state->quad_to_target_transform, quad_damage_rect);
+    // Convert the quad damage rect into its target space and clip it if
+    // needed. Ignore tiny errors to avoid artificially inflating the
+    // damage due to floating point math.
+    constexpr float kEpsilon = 0.001f;
+    gfx::Rect rect_in_target_space =
+        cc::MathUtil::MapEnclosingClippedRectIgnoringError(
+            quad->shared_quad_state->quad_to_target_transform, quad_damage_rect,
+            kEpsilon);
     if (quad->shared_quad_state->is_clipped) {
       rect_in_target_space.Intersect(quad->shared_quad_state->clip_rect);
     }