feat: add fence for shared texture memory

[pepperoni505]

Closes #369

Adds GPUSharedFence, exposing Dawn's shared-fence primitives so GPUSharedTextureMemory can be synchronized across queues/APIs. Until now beginAccess/endAccess only took the no-fence path, which is fine for single-producer still frames, but it can't safely coordinate an external producer and a GPU consumer on different queues (e.g. a video/camera surface being written while Dawn samples it).

[wcandillon]

I like it a lot, thank you for the PR.

In my case I was flip flopping between 1. enabling people to do everything in JS using the SharedTextureMemory API or 2. pushing the importExternalTexture API.

And I realized that it's both 1. and 2.

Item 1 makes a lot of sense considering that this is React Native. You want to give full access/control over the native APIs. And I have a list of items there and this PR is part of that list, so again thank you for doing this.

And item 2 is also unavoidable anyway for zero-copy YUV texture support on Android.
So now my mental model on how to approach this is a bit tidier than yesterday.