Before this change, GrStencilAndCoverTextContext would send 6-float
affine transforms to drawPaths for every glyph. This updates it to
concat the text scale onto the context matrix, and then only send
2-float translates (or 1-float x-translates when possible).
Also adds a glyph_pos_align test to gm that exercises the newly added
codepaths, and starts ignoring a few gm tests with benign pixel diffs
until we can rebaseline.
BUG=skia:
R=bsalomon@google.com, kkinnunen@nvidia.com, jvanverth@google.com, bungeman@google.com
Author: cdalton@nvidia.com
Review URL: https://codereview.chromium.org/406523003
