Build changes I needed for Shotcut on Windows

I am now successfully making nightly builds of Shotcut with WebVfx using my fork. These are some build-only things I needed for Windows. I think you might want to skip the commit "Shotcut-specific static build of libwebvfx on Windows." I ran into some problem with webvfx.dll and libstdc++ that I banged my head against for a couple of days. So, I ended up just statically compiling webvfx and linking mltwebvfx.dll against that.

P.S. I am looking at reusing the Qt labs htmleditor as a titler in Shotcut with superscrollorama for animation.