Detect failed DirectWrite fonts and fall back to GDI correctly

[kleuter]

On some systems (Windows 7/8.1) Qt fails to initialize DirectWrite fonts database and all text is broken (see screenshots below).

Users reported of warning at QWindowsDirectWriteFontDatabase::populateFontDatabase():

Can't initialize IDWriteFactory6. Use GDI font engine instead.

The current workaround is run your app with the "-platform windows:nodirectwrite" command line or add the following to qt.conf:

[Platforms]
WindowsArguments = fontengine=freetype

Looks like Qt doesn't seem to do correct fallback automatically. This should be fixed.

Issue on Windows 7:

Issue on Windows 8.1: