Make use of VkSemaphores thread safe assuming use of the same VkQueue.

This allows us to re-enable support for multiple GrContexts in
GrBackendTextureImageGenerator.

Bug: skia:
Change-Id: Ifd6ac1ad81cdfbd1fd986467d8beb359399d6588
Reviewed-on: https://skia-review.googlesource.com/98340
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
17 files changed