With a large number of tracks, the initial load is very slow

I accidentally captured a profile with 1800 threads. It took a long time before the UI was visible. Here's a profile of the profiler while it was hanging: https://share.firefox.dev/34O2gSO
(I am not sharing the 1800 threads profile.)
I can see the following problems:

* Layout thrashing: For each canvas we add to the timeline, we seem to do a separate layout flush.
* Too many resize listeners: Every canvas has its own window resizing listener.
* Drawing invisible canvases: It would be good to only draw the canvases that are visible.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-395)