Move glyphMargin() to QFontEngine

Move glyphMargin() to QFontEngine

glyphMargin() support for QTextureGlyphCache is implemented in
respective font engines, thus this function is platform dependent.
Before Qt 5 the code is guarded in macros like #ifdef Q_WS_MAC,
now we should move them into QFontEngine and its subclasses.

So far only Windows font engines support it. FreeType and Core Text
based font engines all ignore it.

Change-Id: Ia14016533d8fbfaacf848a7d3bc928f8197318f5
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>