Set a limit on the number of WEBMs transcoding at once

[moffatman]

Starting too many WEBM transcoding sessions at once will probably crash the device. With hardware transcoding I believe it throws an error when too many are added, so it is not fatal. The queue would need to reorder, it would be unacceptable to have to load all preceding WEBMs before the final ones. Could be an issue for #21