have canvas send discard instead of retain if the draw would overwrite everything

have canvas send discard instead of retain if the draw would overwrite everything

Motivation:
- perf win for clients that overwrite the surface after a snapshot.
- may allow us to eliminate SkDeferredCanvas, as this was its primary advantage.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/c83a29759a5c2966da5ab973e4fd90763e4c962b

[reed1, 2015-07-15 15:19:56]

reed@google.com changed reviewers:
+ junov@google.com, robertphillips@google.com

[reed1, 2015-07-15 15:20:14]

incomplete, but wanted reviewers to see the architecture

[Justin Novosad, 2015-07-15 18:00:46]

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

[Justin Novosad, 2015-07-15 18:00:46]

https://codereview.chromium.org/1236023004/diff/20001/src/core/SkCanvas.cpp
File src/core/SkCanvas.cpp (right):

https://codereview.chromium.org/1236023004/diff/20001/src/core/SkCanvas.cpp#n...
src/core/SkCanvas.cpp:48: bool SkCanvas::willOverwriteAllPixels(const SkRect*
rect, const SkPaint* paint,
"will" is ambiguous and can be interpreted as a declaration ("I am going
to..."). I would suggest using a name that makes it obvious that this function
is a question. Perhaps a name that starts with "is".

https://codereview.chromium.org/1236023004/diff/20001/src/core/SkCanvas.cpp#n...
src/core/SkCanvas.cpp:143: if (fSurfaceBase->outstandingImageSnapshot()) {
+1  This is a an interesting optimization

[reed1, 2015-07-15 21:41:02]

Justin, do you think this sort of change will allow us to remove
SkDeferredCanvas (and its associated subclass in blink::ImageBuffer)?

https://codereview.chromium.org/1236023004/diff/20001/src/core/SkCanvas.cpp
File src/core/SkCanvas.cpp (right):

https://codereview.chromium.org/1236023004/diff/20001/src/core/SkCanvas.cpp#n...
src/core/SkCanvas.cpp:48: bool SkCanvas::willOverwriteAllPixels(const SkRect*
rect, const SkPaint* paint,
On 2015/07/15 18:00:46, Justin Novosad wrote:
> "will" is ambiguous and can be interpreted as a declaration ("I am going
> to..."). I would suggest using a name that makes it obvious that this function
> is a question. Perhaps a name that starts with "is".

Done.

[reed1, 2015-07-15 21:56:06]

The CQ bit was checked by reed@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-07-15 21:56:25]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1236023004/60001

[reed1, 2015-07-15 21:59:05]

ptal

[commit-bot: I haz the power, 2015-07-15 22:03:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-15 22:03:59]

Dry run: This issue passed the CQ dry run.

[reed1, 2015-07-16 13:43:26]

ptal

[reed1, 2015-07-16 14:11:15]

reed@google.com changed reviewers:
+ fmalita@chromium.org

[Justin Novosad, 2015-07-16 14:34:32]

On 2015/07/16 14:11:15, reed1 wrote:

lgtm

[reed1, 2015-07-16 14:39:45]

The CQ bit was checked by reed@google.com

[commit-bot: I haz the power, 2015-07-16 14:39:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1236023004/60001

[commit-bot: I haz the power, 2015-07-16 14:40:49]

Committed patchset #4 (id:60001) as
https://skia.googlesource.com/skia/+/c83a29759a5c2966da5ab973e4fd90763e4c962b

[robertphillips, 2015-07-20 15:15:30]

posthumous typos

https://codereview.chromium.org/1236023004/diff/60001/include/core/SkCanvas.h
File include/core/SkCanvas.h (right):

https://codereview.chromium.org/1236023004/diff/60001/include/core/SkCanvas.h...
include/core/SkCanvas.h:1366: // can perform copy-on-write or invalidate any
cached images
willOverwriteEntireSurface (no extra 's') ?

https://codereview.chromium.org/1236023004/diff/60001/src/core/SkCanvas.cpp
File src/core/SkCanvas.cpp (right):

https://codereview.chromium.org/1236023004/diff/60001/src/core/SkCanvas.cpp#n...
src/core/SkCanvas.cpp:137: 
same typo here

https://codereview.chromium.org/1236023004/diff/60001/src/core/SkCanvas.cpp#n...
src/core/SkCanvas.cpp:149: SkSurface::ContentChangeMode mode =
SkSurface::kRetain_ContentChangeMode;
complete -> completely