Investigate performance issues with recording replay canvas

[billyvg]

Example profile of this occurring.

We should look into moving the createImageBitmap call into a webworker.

Another task that blocks the UI is on the initial serialization snapshot. rrweb calls toDataURL() on the canvas in the main UI thread to export as an image.

[bruno-garcia]

We'll potentially move the createImageBitmap call to the web worker. Further investigation needed

[billyvg]

• We can't move createImageBitmap to web worker because:
  • we would need to transfer canvas to webworker which can only be done if we convert it to offscreen canvas
  • by transferring control to offscreen canvas, we cannot write to the canvas from the UI
  • it might be possible to clone the canvas and transfer it to web worker -- initial benchmarks seem to indicate that copying the canvas is faster than createImageBitmap, though probably increased memory usage

Though, from looking at recent profiles, I don't see createImageBitmap as that big of an issue -- I see more main thread blocking from toDataURL when serializing the initial DOM snapshot. Have not gotten around to investigating that yet.