Automate font style matcher fallback font fitting

[Munter]

Given the traced font information, implement automation of what font style matcher lets a human do.

This could be useful for future automation of injection of a fallback for system fonts that avoids font metric jumps when webfonts load late. See the technique presented in https://www.youtube.com/watch?v=tO01ul1WNW8

Possible techniques for a heuristic that can approximate the different font settings:

• For all visible letters, compare the rendered webfont with different local fonts in different sizes to approximate the best fitting fallback font and its font-size.
• For all sentences, compare the length of the spaces between words to approximate the best word-spacing.
• For all sentences (or just the longest one), use a fitting of the X-coordinate of the right edge of the last letter to approximate the best word-spacing.
• For all sentences (or just the longest one), compare the line-wrapped renderings of the original and fallback to approximate the best line-height

Maybe we should use a canvas and overlay the two fonts, like font style matcher does, then fit on the least amount of discrepancy between the font outlines, determined by the lest color difference

[papandreou]

Made such a thing here: https://github.com/papandreou/font-matcher

The outstanding issue is to figure out how to make a minimal set of CSS rules and exactly how to apply them until the original font(s) have loaded.