IBA::paste bug fix, oiiotool --paste, oiiotool --mosaic

Minor bug fix that made IBA::paste() misbehave when the foreground extended over the edges of the background image.

Although we had IBA::paste, and used it in several places, we didn't directly expose it.  So now we do: 
    oiiotool --paste <pos>

Final goody: oiiotool --mosaic WxH takes w*y images and assembles them into a mosaic image, with optional black padding.