[graphite] Add flag to mark Plots as 'full'.

Because Vello always clears when it draws a scene, we can't
incrementally add to a Plot once the initial dispatch is sent. So for
the moment we'll use this to mark a Plot as in use so we don't
accidentally overwrite it.

Bug: b/305696981
Change-Id: Ib03c44061fdcee4c39876c1592d33e9c5093e53c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/848216
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed