Cannot merge 4D virtual stacks

Or maybe any virtual stacks... needs further testing.

See [this forum post](http://forum.imagej.net/t/merging-virtual-hyperstacks/225).

It might depend on how the virtual stacks are created, since not all virtual stacks are created equal—Bio-Formats makes them differently than SCIFIO does, which makes them differently than ImageJ1's built-in functions such as `TIFF Virtual Stack...`.

Here is a macro that creates a couple of virtual stacks using SCIFIO:

``` javascript
many = "many-planes&lengths=512,512,5000,10&axes=X,Y,Z,Time.fake";
lots = "lots-more&lengths=512,512,5000,10&axes=X,Y,Z,Time.fake";
run("Image... ", "source=" + many + " crop=false groupfiles=false mode=Cell");
run("Image... ", "source=" + lots + " crop=false groupfiles=false mode=Cell");
```

You can then run the `Merge Channels...` and see that things do not merge as expected.

Start by running the supplied JavaScript macro to create the two SCIFIO virtual stacks, then use Merge Channels... and compare the result with the expected stack dimensions and contents. Test the same behavior across the virtual-stack creation methods mentioned: Bio-Formats, SCIFIO, and ImageJ1's TIFF Virtual Stack....

Written by the indexing model from the issue text.