[skrifa] assess autohinting performance

[dfrg]

Testing with fauntlet revealed that applying autohinting to the Handjet font takes considerable time to the point where we should likely avoid doing it at all.

Let's get some metrics for various fonts (particularly on mobile) and try to come up with reasonable limits to avoid performance cliffs.

Note: cases where autohinting takes excessive time likely indicate very complex or overly dense geometry which will either not benefit from hinting or produce distorted results.