font_family_without_font_face (and system_font_will_alias) collect
@font-face blocks with `@font-face\s*\{[^}]*\}`, which stops at the first
`}`. A CSS comment inside the block that contains a brace —
`@font-face { /* 400 } regular */ font-family: 'X'; ... }` — truncates the
match before the font-family, so the family is never recorded as declared
and a later `font-family: 'X'` usage is wrongly flagged as used without an
@font-face. Strip CSS comments before scanning so a brace inside one cannot
split a block.
Refs #1534
