Fix skottie2movie gpu async crash

We need to ensure all async reads are completed before we finish
encoding -- otherwise late reads will fire from the GrContext destructor
and crash because the encoder is no longer valid.

Change-Id: I0065561d2411ddd202897e625a187592e40ced87
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447184
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
1 file changed