Revert "usbcam: Introduce PacketFrame to extend life of AVPacket until after write_frame has been called."

This reverts commit 14abbd3598d5bb5191b715fc2a7cea1881c80e06.
ffmpeg: Refactor tying packet to frame code.
ffmpeg: Remove FILE, DEVICE, STREAM syntax in favour of the uri style only.
ffmpeg: Use dshow:// rather than device:// to use a usb camera.