Raster display item lists via a visual rect RTree.

cc/proto/display_item.proto

Index: cc/proto/display_item.proto
diff --git a/cc/proto/display_item.proto b/cc/proto/display_item.proto
index 930c16664e1a6f051b46296c7233056d4bba5ac8..83a2a794c6de06a0437113ffce3cf8e739251632 100644
--- a/cc/proto/display_item.proto
+++ b/cc/proto/display_item.proto
@@ -21,8 +21,10 @@ message DisplayItemListSettings {
 
 message DisplayItemList {
   repeated DisplayItem items = 1;
+  // Deprecated.

[vmpstr, 2016/06/22 20:14:07]

I think at this point in time, we're free to remove things as need +nyquist@ for
an opinion

[nyquist, 2016/06/24 18:31:00]

I think that right now it's fine to remove this field. I'd still add the new
field with a new ID like you've done though.

[wkorman, 2016/06/24 19:57:14]

OK, removed. There are varying opinions on best practices to remove a proto
field even when in nascent usage, I've just literally deleted its entry here.
Some would say to leave it commented out for history, others would say to leave
it with a prefix like DELETED. Open to whatever is preferred.

[vmpstr, 2016/06/24 20:15:33]

I agree with deprecating fields in live code, since there would be clients in
existence that depend on the field. My recommendation here is only because this
is not launched yet, so we might as well do all the cleanup we can before it's
released. 

   optional cc.proto.Rect layer_rect = 2;
   optional DisplayItemListSettings settings = 3;
+  repeated cc.proto.Rect visual_rects = 4;
 }
 
 message DisplayItem {