WebGL: No resize, scaling, screenshot, or HiDPI support, and linear filtering is used

I've been on a deep dive in WebGL and there is a fair amount of missing functionality.

I've managed to implement the aforementioned missing features on a separate branch.

It'd be great to integrate the features into the main repo.

This is the branch: https://github.com/jasonjmcghee/bracket-lib/tree/issue_254_webgl_missing-features

([Here's an link for the comparison](https://github.com/amethyst/bracket-lib/compare/master...jasonjmcghee:issue_254_webgl_missing-features?expand=1))

If there is support here / buy-in I'm happy to add comments / documentation to ensure everything is abundantly clear, or test coverage anywhere that makes sense. I've just been following existing patterns for this code.

## So - what changed?
- Added automatic resize support
- Added scaling support
- Added HiDPI monitor support
- Added ability to take a screenshot
- Use NEAREST filtering + CSS `pixelated` and equivalent to make super sharp visuals

Note: On the branch I have a number of CSS changes on the `index.html` itself - no reason it couldn't be all rust.

# Before
(only using my fix in #251, to fix the black screen):

Generally how it looks (locked at 640x480, with no HiDPI):

Zoomed way in (blurry):

# After
This is taken with the new screenshot method :)
(make sure to click the image - 0 blurriness)

This is zoomed way in:

Scaling / Auto-resize: