ImgSaver: only set rgbCCount if multichannel
ImgSaver: only set rgbCCount if multichannel

The rgbChannelCount variable will now only be set if the output is
actually multichannel (e.g. isMultichannel returns true). Otherwise, we
will get AIOOB exceptions due to attempting to write RGB data to non-RGB
planes.

scifio/scifio