Issue #17321. Under Linux, using "font-family: sans-serif" previously
caused Servo to select the "UltraLight" face (of DejaVu Sans). There
were two reasons for this:
1. Font weight was only retrieved from the OS/2 table for bold faces.
This neglected to retrieve the weight information for "lighter than
normal" weight faces. This meant that the UltraLight face appeared as
normal weight, and was selected.
2. Retrieval of font stretch information from the OS/2 table was not
implemented at all.
Previously anything with the wrong stretch/italicness was considered
equally bad. Now we consider the wrong weight to be least bad, wrong
boldness of intermediate badness, and wrong itallicness to be most
bad.
`FontContext::get_layout_font_group_for_style()`.
There are several optimizations here:
* We make font families atoms, to allow for quicker comparisons.
* We precalculate an FNV hash of the relevant fields of the font style
structure.
* When obtaining a platform font group, we first check pointer equality
for the font style. If there's no match, we go to the FNV hash. Only
if both caches miss do we construct and cache a font group. Note that
individual fonts are *also* cached; thus there are two layers of
caching here.
15% improvement in total layout thread time for Facebook Timeline.
