[graphite] Include offset in Writers returned by DrawBufferManager
[graphite] Include offset in Writers returned by DrawBufferManager

Previously, while BindBufferInfo had the right offset for binding the
buffers, the writer would always overwrite the contents at the
beginning. Never encountered it because the CommandBufferTest made its
VertexWriters directly, and DrawPass isn't end-to-end yet.

Bug: skia:12466
Change-Id: Ib874006d80a7286fcc3710cd38b3c6141ff928ef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/474057
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>
Auto-Submit: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Reviewed-by: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>

aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia