[graphite] Handle failed buffer mappings in DrawWriter

This is an alternative to https://skia-review.googlesource.com/c/skia/+/841836
to reduce the number of if checks (per append() instead of per
attribute write), and hopefully recover some of the 7kb code size
increase caused by the CL.

Change-Id: I0f97f9c3015873a6cd9c80196e16d73f44993443
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/844338
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
7 files changed