Parallel encoding of channels

Currently each channel of a source is video/image encoded sequentially on one CPU. Some of the encoders already use all cores, others use hardware encoders, but each channel could operate in separate threads and this may improve performance slightly. Channel 0 can stay on original encoding thread and if there is a second channel it could create a thread pool job before channel 0 starts encoding, the channel 0 thread then waits for the channel 1 thread to complete.