Instead of applying a transform for every RenderObject, pass down an Offset for where to paint.

Instead of applying a transform for every RenderObject, pass down an Offset for where to paint.
This should be a perf win, but I haven't tested it.

Also adds an operator overloading such that Offset + Size -> Rect.
Also adds Rect.fromLTWH().

R=abarth@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/136e0d4da5c8cfb802c7b77fb811a5b5d1293003

[abarth-chromium, 2015-06-27 00:39:36]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2015-06-27 00:39:38]

LGTM.  Much cleaner than I was expecting.

https://codereview.chromium.org/1209413004/diff/1/sky/engine/core/painting/Of...
File sky/engine/core/painting/Offset.dart (right):

https://codereview.chromium.org/1209413004/diff/1/sky/engine/core/painting/Of...
sky/engine/core/painting/Offset.dart:27: return new Rect.fromLTWH(dx, dy,
other.width, other.height);
Wow, an offset + a size is a rect?  Crazy.

https://codereview.chromium.org/1209413004/diff/1/sky/sdk/lib/rendering/box.dart
File sky/sdk/lib/rendering/box.dart (right):

https://codereview.chromium.org/1209413004/diff/1/sky/sdk/lib/rendering/box.d...
sky/sdk/lib/rendering/box.dart:653: Rect rect = offset + size;
This looks really slick, but I wonder if it's going to be confusing later. 
Maybe operator* would be clearer?

https://codereview.chromium.org/1209413004/diff/1/sky/sdk/lib/rendering/box.d...
sky/sdk/lib/rendering/box.dart:1113: canvas.translate(-offset.dx, -offset.dy);
It's a shame that custom paint callbacks need to pay this penalty.  It's
probably worth it for programming simplicity though.

https://codereview.chromium.org/1209413004/diff/1/sky/sdk/lib/rendering/parag...
File sky/sdk/lib/rendering/paragraph.dart (right):

https://codereview.chromium.org/1209413004/diff/1/sky/sdk/lib/rendering/parag...
sky/sdk/lib/rendering/paragraph.dart:186: canvas.translate(-offset.dx,
-offset.dy);
Ouch.  Can you add a TODO about teaching layoutRoot about offset?  I'm sure the
underlying C++ code can handle it.

[Hixie, 2015-06-29 16:07:17]

Committed patchset #1 (id:1) manually as
136e0d4da5c8cfb802c7b77fb811a5b5d1293003 (presubmit successful).