Update glyph cache type/format logic to match qtbase changes

https://codereview.qt-project.org/#change,77791 unified the glyph format
between QFontEngine and QFontEngineGlyphCache.

Now that we're up to date with qtbase, we can unpin the dependency.

Change-Id: Ibcafccb741ce94b454ff7c469804f44fef7fe06f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>