Inconsistencies when rendering condensed fonts on different platforms

When rendering a condensed font and setting a stretch, the interpretation of this is different in different font engines.

We decided on the following:
A. If QFont::stretch() is set to 0 == AnyStretch (the default), then the stretch of the selected font will be used unmodified.
B. If QFont::stretch() is set to the same stretch as the selected font then we will also just use the font unmodified.
C. If QFont::stretch() is set to something else, then we will synthesize this based on the stretch of the selected font (so if QFont::stretch() returns 40 and the font is condensed at 80, then we will synthesize a stretch of 50)

This currently works as expected in the Mac engine, but only when font merging is enabled.

On Windows GDI, Windows DirectWrite and Freetype, it does not seem to work at all.