QTextObjectInterface assumes no descent

When widgets and quick use QTextObjectInterface to draw an object it interprets the intrinsic height of the object to be all ascender. 

For widgets this is not so noticeable, as the painter isn't clipped, so you can draw outside of the rect you're given, but with quick the object is drawn into a temporary QImage the same size the object reported.

This can be worked around by setting an explicit vertical adjustment on the QTextCharObject to QTextCharFormat::AlignBottom, but this will break if the user overrides that for a section of the text, or the line height of the line is different than for the inserted object.