Fix crash in FrameStore

[kean]

I don't know if that's an ideal design, but it resolves #173. I tested it and I no longer see Thread Sanitizer warnings.

By the way, does this work need to be performed in a background?

[kean]

This should also resolve #132

[kaishin]

@kean Thank you for the PR! I didn't manage to look into #173 and I appreciate you taking a look.

I will go over the changes but as you mentioned it might be an unnecessary optimization to perform this in a background thread. Some performance benchmarks might be necessary.

[kaishin]

Locks are indeed a good solution for the time being. Might need to rethink a bit how the animator and frame store are designed with concurrency in mind.

Preparing frames on the main queue didn't seem to affect performance in the demo app but there is a need for a more demanding example before making that call.

Let's get this in for now and revisit later!