vello_gpu: reuse staging belt for strips upload

[HigherOrderLogic]

Clear more TODO.

This remove the queue allocation and use an arena-like staging belt instead.

[LaurenzV]

Ah, sorry, I forgot to mention that there was a reason why we haven't used the wgpu-native staging belt, see this PR: #1532

Or does this problem not exist in this implementation?

[HigherOrderLogic]

Or does this problem not exist in this implementation?

I think it should eliminate the overlapped memory problem, since the old belt is dropped every time a new one is created. However, there would still be overlapped memory spike, for example when frames are submitted too fast, but this should only last for a short duration.