Auto merge of #11946 - emilio:animations-sender, r=bholley

style: Remove the Mutex from new_animations_sender by moving it to the local StyleContext.

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As a follow-up, we could move all the data living under a mutex in the
SharedLayoutContext only in order to create the local context to the same place.

This should increase animation performance when there are multiple animations in
one page that happen to be on different threads.

r? @SimonSapin/@mbrubeck for the style/layout, @bholley for the geckolib changes

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