Fix coretext shaper on how to decide fallbacks happen

coretext shaper returns all 0 glyph IDs even for non-fallback fonts.
It's cause it currently compares the CGFont copied from the CTRun
with the CGFont used when creating the original CTFont, a technique
shown in [1]. However, CFEqual() seemed to return false even for
identical fonts getting from single CTRun without any fallback
happening. [2] does show a different method, i.e. comparing the
CTFonts directly instead, which seemed to work for me. (On OS X 10.9)

[1] https://github.com/WebKit/webkit/blob/master/Source/WebCore/platform/graphics/mac/GlyphPageTreeNodeMac.cpp#L134
[2] https://github.com/WebKit/webkit/blob/master/Source/WebCore/platform/graphics/mac/ComplexTextControllerCoreText.mm#L249