Fix SDF font positions when using global scale.

Also fixes a crash in the dftext GM when using SampleApp,
and adds new test case in dftext.

BUG=skia:2928
R=joshualitt@google.com, egdaniel@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/588223002
3 files changed