The idea here is to have a central place that does layout for drawText(), and
then always feed text through drawPosText(). This both makes all of the
GrTextContexts consistent in drawText() output, and does a better job of
stressing drawPosText().
Because of the effect of matrices on hinting and approximation error, the
generated text is not 100% identical to that produced by the raster pipeline.
BUG=skia:2778
Review URL: https://codereview.chromium.org/653133004