cache private readback for gpu-images

cache private readback for gpu-images

Does not try to cache calls to readPixels at the moment:
- not triggered by drawing
- not clear if we want to perform any pixel transformations (that readPixels allows) on the GPU or CPU

Can consider that another time.

BUG=513695
Committed: https://skia.googlesource.com/skia/+/6f1216ac158e36a3a1cc805e7f899c755c5b98a2

[reed1, 2015-08-03 21:57:20]

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

[reed2, 2015-08-03 23:57:40]

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

[reed2, 2015-08-03 23:57:41]

still to do:

- write unittest for new cache methods
- preemptive purge from cache when image goes away
- tunnel imageID to be pixelRef's ID

[reed1, 2015-08-04 14:56:39]

ptal

[reed1, 2015-08-04 14:56:45]

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

[commit-bot: I haz the power, 2015-08-04 14:56:58]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1262923003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1262923003/20001

[reed1, 2015-08-04 14:57:22]

The CQ bit was unchecked by reed@google.com

[bsalomon, 2015-08-04 15:02:58]

lgtm

[reed1, 2015-08-04 15:03:10]

The CQ bit was checked by reed@google.com

[commit-bot: I haz the power, 2015-08-04 15:03:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1262923003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1262923003/20001

[commit-bot: I haz the power, 2015-08-04 15:10:16]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/6f1216ac158e36a3a1cc805e7f899c755c5b98a2