Initial Version of Retina Scaling

This adds a new function and variable to Morphic.
Variable: `PIXEL_RATIO` represents the browser's internal scaling or canvas's.
`retinaScale(canvas)` will scale the canvas based on the pixel ratio's the
browser is using.

Right now, only the main `world` canvas is scaled, which still represents a nice
improvement -- individual lines are much sharper. However, text and some
graphics are still a bit blurry. It seems like modifying `newCanvas()` to scale
a canvas when it's created is the right move, but this leads to poor scaling and
many elements appearing larger than they should.

(I also replaced the tabs with spaces when I modified snap.html since that's
what my editor did...and it's consistent with the JS files.)