Unify glyph format between QFontEngine and QFontEngineGlyphCache

Instead of the glyph cache having its own cache type that always mapped
one to one to a font engine glyph format, causing confusion and needless
conversions, the glyph caches now use QFontEngine's glyph format enum.

This also removes the iffy use of an int for the glyphFormat in the font
engines.

Change-Id: I529bad5c179e004f63e152f7dcc311d298c3db98
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>