We should stop treating floating ::first-letter frames as floats using our standard float code. I think the designers of CSS (in this case, Håkon Lie) intended floating first-letter to be "like a float" but not "exactly a float".
* make floating :first-letter participate in inline layout
* use the bounding metrics of the glyph(s) to size the frame
* give it a special hard-coded 'vertical-align' (probably roughly 'text-top')
* make it not participate in line-height calculations
* mark the space as occupied in the space manager