Preserve ordering of font-* properties that follow a font shorthand

[samizdatco]

This PR prevents non-shorthand font-* properties being reordered to precede a shorthand font rule.

Existing behavior

As described in #1207, the FontHandler currently watches for all the properties that could be combined into a shorthand (e.g., font-family, font-weight, font-style, etc.), but will immediately emit any unrecognized font properties such as font-variant or font-kerning while deferring the font shorthand.

For example (playground):

.foo{
  font: 12px serif;
  font-variant-numeric: oldstyle-nums;
  font-kerning: none;
}

will be minified to:

.foo{font-variant-numeric:oldstyle-nums;font-kerning:none;font:12px serif}

This is a problem because placing the font shorthand at the end has the side-effect of resetting all prior font-* properties to their defaults, which is clearly not the intent of the original css.

Changes

I added a new match branch to FontHandler’s handle_property method to catch any Property::Custom that begins with "font-" (but has not already been caught as a property that could be merged into the shorthand) and cache it in a longhand_properties vector. The cached properties can then be flushed after the shorthand.

Initially I applied this to just the font-variant-* properties (cd12e58), but I believe this behavior is actually desirable for all non-shorthand font properties so I extended it to the whole font-* prefix.

Fixes #1207

Questions

I'm not quite clear on the distinction between Property::Custom and Property::Unparsed; maybe this needs to check Unparsed properties to see if they match the font-* prefix as well?

I added an optimization that discards the cached longhand properties when a shorthand font declaration is encountered (since they'll all be reset to their defaults anyway). Are there any negative consequences to doing this?