spanification: Add SK_UNSAFE_TODO in gpu/graphite (1/7)

As part of the effort to enforce -Wunsafe-buffer-usage across Skia, this
patch inserts SK_UNSAFE_TODO macro in targeted lines that trigger the
warning in a subset of files in the gpu/graphite directory, and deletes
their file level check exclusion in unsafe_buffers_mappings.

Upcoming follow-up work includes:
- Refactoring existing code to use safe constructs (e.g., spans).
- Expanding enforcement across directories beyond gpu/graphite.

This CL introduces no behavior changes.

Automated via: https://paste.googleplex.com/4903479387226112
Design doc: http://go/unsafe-buffer-skia

Bug: b/565401613
Change-Id: I066e76247aa5fdfc4b204b6b0301542d17b92f21
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1375836
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Francisco Ochoa <frankchavez@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>