Add bigtileimagefilter GM & improvements to toString

Add bigtileimagefilter GM & improvements to toString

This is all the ancillary code from https://codereview.chromium.org/1152553006/ (Fix dst bound reported by SkTileImageFilter).

TBR=reed@google.com, senorblanco@google.com

Committed: https://skia.googlesource.com/skia/+/63195181d12751bed2fae6e167c7bed7f4a2de2a

[robertphillips, 2015-06-08 12:57:12]

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

[robertphillips, 2015-06-08 12:57:47]

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

[commit-bot: I haz the power, 2015-06-08 12:57:57]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1169713003/1

[commit-bot: I haz the power, 2015-06-08 13:20:18]

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

[commit-bot: I haz the power, 2015-06-08 13:20:19]

Dry run: This issue passed the CQ dry run.

[robertphillips, 2015-06-08 13:20:40]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2015-06-08 13:20:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1169713003/1

[commit-bot: I haz the power, 2015-06-08 13:21:17]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/63195181d12751bed2fae6e167c7bed7f4a2de2a