CSS Fonts Module Level 4: Support font-style: oblique with an angle

[papandreou]

Working draft: https://www.w3.org/TR/css-fonts-4/#font-style-prop

At the moment it's only supported by Firefox, so it's not really urgent.

Also plays into the font matching algorithm: https://www.w3.org/TR/css-fonts-4/#font-matching-algorithm

Presently font-snapper only works properly with font-style: oblique without an angle. There's a hack in place to avoid crashing:

font-snapper/lib/fontSnapper.js

Lines 7 to 14 in 905eb49

	// Hack to treat oblique <angle> as oblique until we add proper support
	function removeObliqueAngle(fontStyle) {
	if (/^oblique\b/.test(fontStyle)) {
	return 'oblique';
	} else {
	return fontStyle;
	}
	}

Once implemented, add to the fuzz tests by switching to headless Firefox and removing:

• font-snapper/test/fuzz.js

  Lines 151 to 153 in ae0f479

  	if (/oblique/.test(propsToSnap['font-style'])) {
  	propsToSnap['font-style'] = 'oblique';
  	}

• font-snapper/test/fuzz.js

  Lines 166 to 168 in ae0f479

  	if (/oblique/.test(fontFaceDeclaration['font-style'])) {
  	fontFaceDeclaration['font-style'] = 'oblique';
  	}