Render linear synteny view using webworker

[cmdcolin]

Background

The LinearSyntenyView renders all the data in the main thread. It downloads all the data to the main thread using CoreGetFeatures and then renders it on every frame.

This PR

This PR changes the rendering to happen in the web worker

It uses transferControlToOffscreen (https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/transferControlToOffscreen) to let the webworker draw straight to the main thread's DOM canvas

Caveat

This is currently significantly less 'smooth' on scrolling, but this could probably be worked on further, and the effect of not bogging down the main thread with extremely slow rendering probably is worth it

[cmdcolin]

Fixes #4616

[cmdcolin]

Example share link using this branch

https://jbrowse.org/code/jb2/refactor_abstract_canvas_renderers/?config=test_data%2Fconfig_synteny_grape_peach.json&session=share-kS1dLgjWnZ&password=ar5c5

can see the synteny rendering lags a little behind but it doesn't 'feel too bad' from a user experience perspective since the interface stays responsive

[cmdcolin]

marked as draft. will need to incorporate stop tokens and improved synchronization (the renderer should respond with what it rendered to help the main thread confirm that that the rendering is relevant)