Sample(s) for hidpi, ResizeObserver, and fullscreen

[kainino0x]

We should have a sample that demonstrates correct usage of ResizeObserver to create canvases which are hidpi, resize-responsive, and fullscreenable. It should:

• Show a checkerboard pattern to demonstrate the pixel size
• Correctly handle both changing browser scale and changing canvas size
• Let the user easily play with resizing by putting a resize handle on the canvas container div
• Have a fullscreen button (perhaps fullscreening not just the canvas but a div containing some trivial HTML like a text overlay)

See also #249 about using hidpi canvases in more samples, which perhaps this sample should supersede.

[kainino0x]

It could possibly also demonstrate CSS image-rendering options.
The sample image from the CSS spec is a nice test case for scaling-related stuff, so we could use it in this sample.
https://drafts.csswg.org/css-images/#example-30207ff9

[greggman]

done