These methods provide a common way to draw HTML text above a canvas.

The getTextInfo method generates div HTML for text with a given font
style/class and angle, measures the element's dimensions, and saves
everything in a cache.  The drawText method takes the resulting entry,
finishes generating the inline styles necessary to position the div, and
adds the result to a buffer.  The render method dumps the buffer into an
overlay and expires unused cache entries.