Make it possible to custom-paint without an Element.

Make it possible to custom-paint without an Element.

This adds a new abstract 'Canvas' which is similar to SkCanvas.
PaintContext implements Canvas while still having its
own commit() method to cause the paint actions to apply to
the Element for the next frame.
This adds a new PictureRecorder which also implements Canvas
and has an endRecording() method which returns a Picture
(another new interface) which can be held from Dart.

There is also now a rootPicture setter on Document which takes
a Picture and will then make the Document draw that Picture
until changed.

This piggybacks on the existing custom painting system
which adds the painting at background-and-borders paint
time so technically if you both set rootPicture as well as
construct a DOM you will draw the DOM on top of your picture. :)

R=mpcomplete@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/197b907de769c8ddaabc5a8e2fa1d6a0c660e319

[eseidel, 2015-05-13 00:35:47]

eseidel@chromium.org changed reviewers:
+ ianh@google.com, mpcomplete@chromium.org

[eseidel, 2015-05-13 00:39:00]

This probably isn't ready for landing.  But it does actually work! :)

I was just so excited to actually have written c++ code for a change I uploaded
it in celebration.

[Matt Perry, 2015-05-13 17:02:48]

If you know it works, I assume you have a sample sky app using it. Could you add
it to this CL somewhere (examples/ ?)

[eseidel, 2015-05-13 18:01:21]

I think this is ready for review.

[Matt Perry, 2015-05-13 18:33:21]

LGTM

https://codereview.chromium.org/1122423009/diff/40001/sky/engine/core/paintin...
File sky/engine/core/painting/Canvas.h (right):

https://codereview.chromium.org/1122423009/diff/40001/sky/engine/core/paintin...
sky/engine/core/painting/Canvas.h:28: void drawCircle(double x, double y, double
radius, Paint* paint);
Skia takes these params as SkScalars, which are floats, not doubles. Is there a
reason we chose not to use float here?

[Hixie, 2015-05-13 19:20:58]

On 2015/05/13 at 18:33:21, mpcomplete wrote:
> LGTM
> 
>
https://codereview.chromium.org/1122423009/diff/40001/sky/engine/core/paintin...
> File sky/engine/core/painting/Canvas.h (right):
> 
>
https://codereview.chromium.org/1122423009/diff/40001/sky/engine/core/paintin...
> sky/engine/core/painting/Canvas.h:28: void drawCircle(double x, double y,
double radius, Paint* paint);
> Skia takes these params as SkScalars, which are floats, not doubles. Is there
a reason we chose not to use float here?

I'm surprised Skia uses floats. My impression was that we should move everything
towards fixed-point units that are some fraction of a "CSS pixel".

[eseidel, 2015-05-13 19:23:34]

Nope.  Happy to change to float.  Thanks!

[eseidel, 2015-05-13 20:16:52]

Committed patchset #3 (id:40001) manually as
197b907de769c8ddaabc5a8e2fa1d6a0c660e319 (presubmit successful).