Reviewed by Eric. Older parts reviewed by Dan.

Fixes: http://bugs.webkit.org/show_bug.cgi?id=16880 (SVGCSSFontFace should die, instead integrate within the FontCache.)
Fixes: http://bugs.webkit.org/show_bug.cgi?id=16784 (In-document fonts work only as the first child to font-face-src)

Proper integration of SVG Fonts within the existing custom font concept, removing the need for a custom SVGCSSFontFace.
Integrate within the CSSFontFaceSource logic for caching support, as well as the framework for external SVG Fonts.

Proper handling of missing glyphs, using <missing-glyph> element. If that is not specified in a font, fallback to non-SVG font rendering.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@29700 268f45cc-cd09-0410-ab3c-d52691b4dbfc