Formulas at the right size

A follow-up to the last release, which taught Floati to draw math equations. Now they render at the right size too. A large encyclopedia sizes its equation images in a typographic unit (ex, tied to the font's height) that Floati did not understand, and it only read an image's size from old-style width and height attributes, not the modern CSS the equations actually use. Both are fixed, so a formula like the Pythagorean theorem now sits inline at its true proportions instead of a stretched box. Also fixed: boxes that set a height and an aspect ratio (a common way to reserve space for images and video) now compute their width correctly instead of collapsing to a sliver.