[Interest] Qt6–QtMultimedia – MinGW Support

[Marius Kittler]

Hi,

I've already built Qt 6 Multimedia with the a recent version of mingw-w64 
(9.0.0) and GCC (11.2.0) but the resulting build was rather limited. In fact 
I've got nothing:

```
Qt Multimedia:
  ALSA ................................... no
  GStreamer 1.0 .......................... no
  Video for Linux ........................ no
  PulseAudio ............................. no
  MMRenderer ............................. no
  AVFoundation ........................... no
  Windows Media Foundation ............... no
```

The problem is
```
-- Could NOT find WMF (missing: WMF_MF_LIBRARY WMF_MFUUID_LIBRARY 
WMF_MFPLAT_LIBRARY WMF_MFCORE_LIBRARY WMF_PROPSYS_LIBRARY) 
```
and I suppose that's really a limitation of mingw-w64 at this point. 
Unfortunately with Qt 6 WMF is the only remaining backend - except for 
GStreamer which I suppose one could also use as an alternative. At least the 
build system was looking for it:

```
-- Could NOT find GStreamer (missing: GStreamer_Core_FOUND 
GStreamer_Base_FOUND GStreamer_Audio_FOUND GStreamer_Video_FOUND
```

I haven't installed GStreamer libs in my build env but if I actually needed Qt 
Multimedia I'd try that. Building GStreamer libs and its dependencies for 
mingw-w64 generally works (maybe not static linking, though).

Maybe there's also an experimental version of mingw-w64 available which has 
support for WMF.