Class: VirtualSourceBuffer

VirtualSourceBuffers exist so that we can transmux non native formats into a native format, but keep the same api as a native source buffer. It creates a transmuxer, that works in its own thread (a web worker) and that transmuxer muxes the data into a native format. VirtualSourceBuffer will then send all of that data to the naive sourcebuffer so that it is indestinguishable from a natively supported format.