Implement CSS Font Matching

[CryZe]

cosmic-text currently implements a very basic 1:1 matching algorithm where if anything doesn't perfectly match, the font will never be chosen:

cosmic-text/src/attrs.rs

Lines 166 to 175 in b9fef72

	pub fn matches(&self, face: &fontdb::FaceInfo) -> bool {
	//TODO: smarter way of including emoji
	face.post_script_name.contains("Emoji") ||
	(
	face.style == self.style &&
	face.weight == self.weight &&
	face.stretch == self.stretch &&
	face.monospaced == self.monospaced
	)
	}

It should implement the CSS Fonts Module Level 3 - 5. Font Matching Algorithm.

Also it's super odd how Emoji is considered the fallback font and not the actual specified fallback font.

[CryZe]

Oh looks like #48 covers this.