Add touchscreen drag/zoom support

[JJJollyjim]

This merge request adds support for full interaction with the canvas through dragging and pinch-to-zoom on mobile devices. It uses the Pointer Events API (a unified touch+mouse API), which is not supported in current Safari without a polyfill (though is in the preview version).

To summarise, without a polyfill:

• Safari loses mouse support
• Android-Chrome gains touch support

And with a polyfill:

• All browsers keep mouse support
• Android-Chrome gains touch support, as do Safari-iOS, Firefox-Android, and desktop browsers.

On the assumption that dropping safari mouse support is an issue, the second commit adds a pointer event polyfill.

I have tested this change (including the polyfill) on:

• Firefox on Linux (mouse still works)
• Chrome on Linux (mouse still works, touch now works)
• Safari on OSX (mouse still works)
• Chrome on Android (touch now works)
• Firefox on Android (touch now works)
• Safari on iOS (touch now works)

[qu1ck]

Thanks a lot for adding touch support. Some comments below.

[qu1ck]

Hey @JJJollyjim please let me know if you will be able to address the review comments soon. I plan on releasing v2.1 soon and would like to get this in but it's ok if you don't have time right now. In that case it will go in v2.2 or whenever it is ready.
Unless I hear from you I will tag v2.1 on Sunday.

[JJJollyjim]

Oh yes I'm busy tonight but will respond tomorrow, thanks for reminding me :)

[JJJollyjim]

I accidentally clicked resolve on the thread about where to center the zooming and can't get it back, but I made a quick video to demo my approach since you mentioned you can't test it: https://youtu.be/g2JyAtq1eE8

In short, zooming on the other finger matches the bahaviour of the OS and doesn't seem to show any shakiness

[JJJollyjim]

I'll push my changes for the style/minifciations comments when I'm back at my computer

[qu1ck]

I think this is in good shape now and I could merge now, take care of touch reset later. Unless you already have some work done?

[JJJollyjim]

Just finished and pushed it :)

[qu1ck]

Thanks for the contribution and patience with code review!