specialize addGlyphToAtlas padding to bilerp case

Change-Id: Id3695ea65b292e39c40a17d039ec4b9347cd0feb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/307296
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
4 files changed