Fix imagemagnifier GM quality and speed.

Fix imagemagnifier GM quality and speed.

The imagemagnifier GM was applying the filter both to a saveLayer()
restore() around the whole scene, as well as to each individual
text draw.

Applying the filter only in the saveLayer() makes it run ~20x faster
on my Linux box, and the quality is improved as well (since the
primitives are not double-filtered).

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/cbf6b6ecd5edca74aa9f78ea5b9a73fb955b8654

[Stephen White, 2014-10-23 21:21:47]

senorblanco@chromium.org changed reviewers:
+ bsalomon@google.com

[Stephen White, 2014-10-23 21:21:54]

Brian: PTAL. Thanks!

[bsalomon, 2014-10-23 21:41:03]

On 2014/10/23 21:21:54, Stephen White wrote:
> Brian: PTAL. Thanks!

geez, lgtm.

[Stephen White, 2014-10-23 21:51:18]

The CQ bit was checked by senorblanco@chromium.org

[commit-bot: I haz the power, 2014-10-23 21:52:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/637283009/20001

[commit-bot: I haz the power, 2014-10-23 22:00:23]

Committed patchset #2 (id:20001) as cbf6b6ecd5edca74aa9f78ea5b9a73fb955b8654