Use default channel layout when not specified by format.
Use default channel layout when not specified by format.

Some file formats to not explicitly specify the channel layout.
Use the number of channels to infer the layout.

Fix for issue 320.

j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt