Break up InitDistanceFieldPaint

Break InitDistanceFieldPaint along code that is common and,
code that is specific to GPU.

Change-Id: Ic831491829fe402a46f4e612c2598d987ae19c8c
Reviewed-on: https://skia-review.googlesource.com/c/193371
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed