[graphite] Add BackendSemaphore and support flag in Caps.
Semaphore support will be needed to get Vulkan working in Viewer.
The Caps flag will be turned on for each backend once it's fully
working in that backend.
Bug: b/286088355
Change-Id: I83b6436f8484bc944ddc2def0e84afe1e3aa90f1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/708120
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
4 files changed