Add standalone drawText for GrTextContext.

This unifies the interface between GrBitmapTextContext and
GrDistanceFieldTextContext so that they don't need special case code.
The future GrNVPRTextContext will also use this interface.

BUG=skia:2018
R=bsalomon@google.com, reed@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/141863005

git-svn-id: http://skia.googlecode.com/svn/trunk@13227 2bbb7eff-a529-9590-31e7-b0007b416f81
9 files changed