lsmashsource: Add 'layout' option.

This option forces output channel layout.
Note: you can't set up mixing configuration by this.

A format of the argument is like the following.
  layout="FL+FR+FC": front left + front right + front center
Also users can specify the same layout as the above as follows.
  layout="3.0"
See libavutil/channel_layout.c/h for more detail.