ImgSaver: correct rgb plane indices

Was mistakenly referring, in SCIFIO, to the imglib plane indices. In the
case of a dataset with more than one RGB plane (e.g. a time series of
RGB data) this would cause plane indices to be off by a multiple of 3 *
their actual plane index.
APNGFormat: RGB read fix

When opening the first plane of an image the APNGFormat reader was
assuming 2 planar axes, which caused an AIOOB if there were more than 2
planar axes (e.g. RGB dataset).

Fixed to use the actual number of planar axes.
ImgSaver: warn when truncating planes

When dimensions are truncated during writing (for example, when saving a
5D dataset to a 3D format) a warning message is now issued, instead of
attempting to write invalid plane numbers (and crashing).