Cross-browser compatibility fixes.

On 64-bit linux, Firefox only seems to support 'experimental-webgl' contexts,
not 'webgl' at this time. Chrome supports both, so I've switched back to using
'experimental-webgl'.

Also, on mouse events, the DOM standard doesn't provide x and y members (these
seem to be a Chrome addition). The correct fields are clientX and clientY
instead.
Add some math helper functions to ludum.js

Added:
- radians, for converting angles from degrees to radians.
- degrees, for converting angles from radians to degrees.
- roundTo, for converting a number to a string wiht a fixed number of decimal
  places.