Windows: Keep using the GDI font engine for now

The new DirectWrite font engine appears to render some fonts blurry.

Valid values for this "fontengine" option are:

- fontengine=freetype
- fontengine=gdi
- fontengine=native

It can also be configured on the command-line using `-platform
windows:fontengine=gdi` for example, or throught an environment
variable, for example `QT_QPA_PLATFORM=windows:fontengine=gdi`.

dist/win/qt.conf

@@ -1,3 +1,6 @@
 [Paths]
 Plugins = plugins
 Translations = translations
+
+[Platforms]
+WindowsArguments = fontengine=gdi