Respect initial stencil contents when merging opsTasks
Respect initial stencil contents when merging opsTasks

Bug: skia:11952
Change-Id: I0eb6f578dcc95fa6e92ff959a6483c7583d0ad9b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/404396
Reviewed-by: Adlai Holler <74c8d5b16bacf4a5997d6535bb4043bbdfe052ea@google.com>
Commit-Queue: Chris Dalton <ef7fc3a08ada7d31f16e9a18b5f3c728256e041e@google.com>

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