This also fixes a bug that the tests revealed -- it wasn't resetting the
reused-buffer offset after transferToCommandBuffer.
Change-Id: I75850ab1cb1e63dc88f95783248f70816611cefc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/536103
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
