ImgSaver: fix slice count calculation

Was comparing Axes to AxisTypes. Fixed to compare type vs. type, as
intended.
ImgSaver: remove unnecessary create ImageMetadata

ImageMetadata will be created automatically.. no reason to explicitly do
so here.
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.