Stream restart to enable DPI changes (mostly complete)

[forderud]

Extend AppWebStream to support DPI change during encoding. The DPI is encoded in the "avc1" atom that's only transmitted at the start of the stream. We'll therefore probably need to re-start the stream to support DPI changes.

Tasks:

• [x] Stream restart with Media Foundation (confirm that the new DPI is actually transmitted)
• [ ] Stream restart with FFMPEG (Libav-user Possible to add new stream to fragmented MP4 after avformat_write_header?)
• [x] Make StreamReceiver detect DPI changes (initial support since 52db72167892fbffec1d371da93b23259ca61e9e)

[forderud]

Initial support added in 9121eb3b55b75696854501f83ebe9126f7c450e3. It's not the most elegant approach though, and FFMPEG implementation is missing.