Get rid of forced reflows inside of the animation loop when calculating size.

[trusktr]

getComputedStyle in Sizeable#actualSize is what causes it. Instead, let's listen to size changes on the root Scene, then we can calculate proportional sizes using plain math without hitting the DOM's getComputedStyle method. This is what we need to do anyways because when rendering to WebGL only (f.e. if DOM rendering were turned off when that option is available in the future), then there may be no DOM elements from which to call getComputedStyle on anyways.

[trusktr]

This will be solved along with #56

[trusktr]

closed in b8672eb