Account for scale factor when we draw an image into a canvas.
Added HtmlGraphics.registerFontMetrics for overriding font line height.

Font measurement in HTML5 is a giant hack, and it's not super surprising that
browsers return wildly different values for the height of a line of text. This
allows a game to override the measured values and just use a value that is
consistent and will actually result in sanely rendered text across browsers.