layout/context: Wrap `image_cache_thread` in a `Mutex<>` as well
`SharedLayoutContext.image_cache_thread` didn't get the `Mutex<>`
treatment along with all the other channels in there, because
`ipc-channel::Sender` is presently inherently `Sync`. I believe this to
be an implementation accident though, that should be rectified in the
future -- not something users should actually rely on...
Note that sharing senders (be it `mpsc` or `ipc-channel`) in is probably
not a good idea anyway: just cloning them -- and letting them handle the
sharing internally -- should be both simpler and cheaper. But right now
that's how things are handled here; so let's go with the flow...
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