Add support for multichannel Mats

Currently we only support conversion from [single-channel Mats](https://github.com/imagej/imagej-opencv/blob/93dec2fe934dc74fc0e96ed0e208995958e34e85/src/main/java/net/imagej/opencv/MatToImgConverter.java#L91-L92). It would be nice to have support for `Mats` with multiple channels.