B-frames should be sent immediately with preceding I- or P-frame


Seek operation call the curr block to get the stream time. But this isn't quite right if the curr block designates a b-frame, since b-frames appear out of order. PopulateSample should check whether the next frame(s) in the stream is a b-frame, and send those immediately, such that pCurr->IsBFrame is always false. (To make this work, we'll also have to ensure that there's large enough buffer in the media sample.)