Let OverlayStrategySingleOnTop select non-topmost quad

Old code was too restrictive, and did not properly select the correct quad.
New code chooses the topmost overlayable plane that does not intersect any
rects above it.

Review URL: https://codereview.chromium.org/440193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288049 0039d316-1c4b-4281-b951-d872f2087c98

cc/output/overlay_strategy_single_on_top.cc

@@ -28,13 +28,38 @@ bool OverlayStrategySingleOnTop::Attempt(
   DCHECK(root_render_pass);
 
   QuadList& quad_list = root_render_pass->quad_list;
-  const DrawQuad* candidate_quad = quad_list.front();
-  if (candidate_quad->material != DrawQuad::TEXTURE_CONTENT)
-    return false;
-
-  const TextureDrawQuad& quad = *TextureDrawQuad::MaterialCast(candidate_quad);
-  if (!resource_provider_->AllowOverlay(quad.resource_id))
+  QuadList::iterator candidate_iterator = quad_list.end();
+  for (QuadList::iterator it = quad_list.begin(); it != quad_list.end(); ++it) {
+    const DrawQuad* draw_quad = *it;
+    if (draw_quad->material == DrawQuad::TEXTURE_CONTENT) {
+      const TextureDrawQuad& quad = *TextureDrawQuad::MaterialCast(draw_quad);
+      if (!resource_provider_->AllowOverlay(quad.resource_id)) {
+        continue;
+      }
+      // Check that no prior quads overlap it.
+      bool intersects = false;
+      gfx::RectF rect = draw_quad->rect;
+      draw_quad->quadTransform().TransformRect(&rect);
+      for (QuadList::iterator overlap_iter = quad_list.begin();
+           overlap_iter != it;
+           ++overlap_iter) {
+        gfx::RectF overlap_rect = (*overlap_iter)->rect;
+        (*overlap_iter)->quadTransform().TransformRect(&overlap_rect);
+        if (rect.Intersects(overlap_rect)) {
+          intersects = true;
+          break;
+        }
+      }
+      if (intersects)
+        continue;
+      candidate_iterator = it;
+      break;
+    }
+  }
+  if (candidate_iterator == quad_list.end())
     return false;
+  const TextureDrawQuad& quad =
+      *TextureDrawQuad::MaterialCast(*candidate_iterator);
 
   // Simple quads only.
   gfx::OverlayTransform overlay_transform =
@@ -69,8 +94,7 @@ bool OverlayStrategySingleOnTop::Attempt(
 
   // If the candidate can be handled by an overlay, create a pass for it.
   if (candidates[1].overlay_handled) {
-    scoped_ptr<DrawQuad> overlay_quad = quad_list.take(quad_list.begin());
-    quad_list.erase(quad_list.begin());
+    quad_list.erase(candidate_iterator);
     candidate_list->swap(candidates);
     return true;
   }