Support devicePixelRatio

[dfabulich]

Fixes #80.

This looks way better on my 16" MacBook Pro. But! It was extremely difficult code to write and work with. Pretty much all of these * dpr and / dprs were discovered through painful trial and error. It wouldn't surprise me at all if I've introduced some subtle bugs, and it seems especially likely that this PR will make the code less maintainable over time.

If your screen doesn't support HiDPI mode, you can manually set App.devicePixelRatio to 2 to get a sense of what it would look like.

• The core approach to devicePixelRatio is to render a larger canvas, but use CSS to resize the canvas down to a smaller than natural size. https://www.html5rocks.com/en/tutorials/canvas/hidpi/ This article describes a common technique, but it doesn't work here, because there's code setting canvas.width and .height based on the CSS width/height 100%. So instead of the normal documented technique, in canvas.ts we're setting the scale to be * dpr and configuring a CSS width: calc(dpr * 100%) and transform: scale(calc(1/dpr))
• App.centerX and .centerY are now proportionally larger, so we have to / dpr in findMouseCoordinates, and * dpr in places where we're setting centerX.

[jeremydouglass]

Nice!! but:

subtle bugs ... make the code less maintainable over time

I love my retina display, but I vote NO on that. Not that I get vote! -- but I really want the project to be maintainable and survive the inevitable need to upgrade and migrate.

[dfabulich]

I think that's an understandable attitude, but if you review the code, you'll see it's not all that bad. Still, I anticipate a bunch of bugs like this in the future: "whoops, I forgot * dpr … or is it / dpr in this case…?"

For me and my eyes, this makes Trizbort look so much better that it's worth wrestling with the bugs.

[henck]

It is indeed a nice feature and it doesn't affect that many places. I'd make it a point to refactor to some sort of CoordCalc class that does the work, and is called whenever a screen coordinate is needed, though.

I've merged this change in.