Disable _onZoomStart in canvas renderer

[klaftertief]

This fixes disappearing paths when starting an animated zoom. See #3947

[IvanSanchez]

Good catch! Seems like I forgot to check the canvas renderer.

Maybe it would be a better idea to call update() only for the SVG renderer instead?

[klaftertief]

That would mean to

• either split up the list of events in getEvents() between the base renderer and the SVG renderer
• or have the _onZoomStart() be a noOp in the base renderer and override it in the SVG renderer.

I'm not sure what would be the cleaner way. I can update the pull request either way.

[IvanSanchez]

I think splitting getEvents would be cleaner, but I'm not 100% sure (no inheritance, etc). If you could try that and see if it looks better, that'd be great.

[klaftertief]

The additional commit looks a bit nicer to me then the splitter getEvents in #3950.

Though I'm not sure if this is really the best fix in general. Panning and then zooming on mobile in the debug/vector/vector-canvas.html test looks works way better in the beta 1 version. There is a slight offset movement on zoom start now.

[IvanSanchez]

Superseded by #3950