The DirectWrite font engine isn't used for OpenType CFF fonts, but it should be

There's much documentation on the web about DirectWrite's superior rendering of OpenType CFF fonts (ie. OpenType fonts with Type 1 PostScript outlines). Our users make extensive use of these professional fonts (eg. all Adobe's fonts) so I was a bit disappointed to perceive no improvements when switching on DirectWrite in Qt 4.8.

I investigated further and discovered that DirectWrite isn't used for non-TrueType fonts, due to logic in loadEngine in qfontdatabase_win.cpp. This checks for TMPF_TRUETYPE in the GDI TEXTMETRIC structure. However, DirectWrite can render any sfnt-format font, whether it be TrueType or CFF, so I think the code should instead check for the presence of a cmap table, like this:

DWORD cmapTag = MAKE_TAG('c', 'm', 'a', 'p');
uint cmapTableLength = GetFontData(hdc, cmapTag, 0, 0, 0);
bool sfnt = cmapTableLength != GDI_ERROR;

and then switch on sfnt to determine whether it should try using DirectWrite. I tried this locally and the results are good.