Manage buffers at the list level only

Now that the painter has moved from a GlyphRun to a GlyphRunList,
handle the buffers at the GlyphRunList level only.

Change-Id: I37416c8b2164bbde207ecab0a77b419775d58e66
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/538659
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed