When using offscreen draws, we save the "previous" frame in fLastImage
in viewer. However this image holds a ref to the GrContext. When we
were destroying the vulkan context we thought we were destroying the
GrContext before the VkDevice but fLastImage was keeping it alive.
This changes makes sure to reset fLastImage when we are done with it.
Change-Id: Ib70e069a261f939217436a9f052d7565581cd230
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/320263
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Auto-Submit: Greg Daniel <egdaniel@google.com>
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