Move GrPorterDuffXferProcessor to include/gpu/effects.

BUG=skia:

Review URL: https://codereview.chromium.org/780263003
get rid of static gps

This may cause a perf regression

TBR=
BUG=skia:

Review URL: https://codereview.chromium.org/749413003
Factor text size device mapping in SkScalerContext.

All of our font back-ends use the text size in some way other than
simple concatentation with the current matrix. The code here finds
the full device matrix and then decomposes it to extract the text
size.

FreeType, GDI, and DirectWrite use the text size as the pre-hint scale.
CoreText will not scale color emoji above the requested text size,
and certain features like 'trak' are performed on the text size.

Review URL: https://codereview.chromium.org/748883005
Fix include for GrPortderDuffXferProcessor.cpp
TBR=bsalomon@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/771353005
rebaseline after GM change

BUG=skia:
TBR=
NOTRY=True

Review URL: https://codereview.chromium.org/784603002
Check XpFactory equality in DrawState

BUG=skia:

Review URL: https://codereview.chromium.org/767873006
Add support for half float alpha textures.

This allows us to create distance field textures with better precision,
which may help text quality.

BUG=skia:3103

Review URL: https://codereview.chromium.org/762923003