ui: Add sub region copy support to GLImage::CopyTexImage.

ui: Add sub region copy support to GLImage::CopyTexImage.

This extends the GLImage API with support for partial copy operations.
GLImage::CopyTexImage gets two new parameters, |offset| and |rect|. The
name of the function is also changed to GLImage::CopyTexSubImage to
reflect the new behavior.

GLImageMemory implementation is updated to optimize copy operations
that span full rows of pixels.

BUG=490889
TEST=gpu_unittests, gl_tests

Committed: https://crrev.com/6b683f23b25fa464663b9c44f032eca0195ef4bc
Cr-Commit-Position: refs/heads/master@{#331051}

[reveman, 2015-05-22 00:39:23]

reveman@chromium.org changed reviewers:
+ piman@chromium.org

[piman, 2015-05-22 01:15:32]

lgtm

[reveman, 2015-05-22 01:25:15]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2015-05-22 01:26:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1152693002/1

[commit-bot: I haz the power, 2015-05-22 02:38:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-22 02:39:59]

Patchset 1 (id:??) landed as
https://crrev.com/6b683f23b25fa464663b9c44f032eca0195ef4bc
Cr-Commit-Position: refs/heads/master@{#331051}