Add a writer class for indirect draw commands

This enforces write-only access to the mapped buffers, will enable
chaining of indirect strokes, and gives us the ability to reorder the
fields for Metal.

Bug: chromium:1172543
Bug: skia:11291
Bug: skia:10419
Change-Id: I4449ff85dd0019f6d6d6781ede52bcf26dee8b02
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/367416
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
15 files changed