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:

Review URL: https://codereview.chromium.org/637283009
2 files changed