Inline SDFTSubRun's implementation into class

Inline all the member function into the class. In addition,
collect all the SK_GRAPHITE_ENABLED and SK_SUPPORT_GPU into
single #ifs. Other small cleanups.

Change-Id: I942c597fee7e2e6cd28430e2730f15dd33e051e1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/589399
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed