Make QFontEngineQPA work to _some_ degree with distance field glyph rendering

QFontEngineQPA doesn't implement cloneWithSize() and thus crashes in qsgdistancefieldglyphcache.cpp. Since QFontEngineQPA typically only has fixed size alpha maps, we should treat those directly as distance fields or compute the distance fields from them, to ease the porting effort when creating a new platform plugin. In the end, the plugin should use the Freetype font engine, so we don't need a perfect solution.