New public method ImageJVirtualStack.getSource returning the underlyi…

…ng RandomAccessibleInterval.

There are many occasions when one wants to get the underlying RandomAccessibleInterval. Offering a public method to return
the private field "source" addresses this well, so one can write, for example:

RandomAccessibleInterval<UnsignedShortType> rai = IJ.getImage().getStack().getSource();

Or more simply from a script:

rai = IJ.getImage().getStack().getSource()

src/main/java/net/imglib2/img/display/imagej/ImageJVirtualStack.java

@@ -222,4 +222,10 @@ protected RandomAccessibleInterval< T > getSliceZeroBasedIndex( int index )
 		}
 		return origin;
 	}
+
+	/** Get the underlying ImgLib2 {@link RandomAccessibleInterval}. */
+	public RandomAccessibleInterval< T > getSource()
+	{
+		return source;
+	}
 }