RGB images sent to ImageJ should use composite mode to preserve color

[ctrueden]

• Open a JPG in napari
• Send it to ImageJ using the advanced transfer button, fixing the dimensions to be correct (YXC)
• Observe that even though the dimensions are correct, the image is still in grayscale, with one channel at a time

ImageJ has a special CompositeImage class to composite multiple channels together at once, and ImageJ2 provides a hint for that, which IIRC is Dataset#setRGBMerged(true). It's unfortunate if napari-imagej needs to end up calling that—this might actually be a bugfix in PyImageJ or even in imagej-legacy somewhere—but I'm documenting the issue here because it's easy to reproduce using napari-imagej.

[gselzer]

I tried to fix this on the rgb-data-transfer branch, but there were errors stemming from imagej-common:

java.lang.ArrayIndexOutOfBoundsException: Index -2 out of bounds for length 1
	at net.imglib2.AbstractInterval.min(AbstractInterval.java:208)
	at net.imagej.Position.setPosition(Position.java:324)
	at net.imagej.Position.setPosition(Position.java:367)
	at net.imagej.display.DefaultDatasetView.getCurrentLUT(DefaultDatasetView.java:556)
	at net.imagej.display.DefaultDatasetView.updateLUTs(DefaultDatasetView.java:543)
	at net.imagej.display.DefaultDatasetView.rebuild(DefaultDatasetView.java:398)
	at net.imagej.display.DefaultImageDisplay.rebuild(DefaultImageDisplay.java:134)
	at net.imagej.display.DefaultImageDisplay.display(DefaultImageDisplay.java:272)
	at org.scijava.display.DefaultDisplayService.createDisplayQuietly(DefaultDisplayService.java:228)
	at org.scijava.display.DefaultDisplayService.createDisplay(DefaultDisplayService.java:210)
	at org.scijava.ui.AbstractUserInterface.show(AbstractUserInterface.java:98)
	at org.scijava.ui.UserInterface.show(UserInterface.java:75)
	at org.scijava.ui.DefaultUIService.show(DefaultUIService.java:242)