Add more efficient way to check existence of font tables

There are a few places in Qt where we currently check the existence of font tables by copying the table and checking its size. This is really inefficient, and we only get away with it because the presence of the tables in question is so uncommon that it will not cause a slowdown in practice, since for almost all fonts it will just be a lookup in the font table directory and return false.

But most platforms should have better ways of doing this. It would make sense to have a virtual bool QFontEngine::tableExists(const char *tag) which can be used for this purpose and which does the check in a platform-dependent way.