Degenerate TileLayouts

[fosskers]

Currently, the following are possible:

scala> TileLayout(0, 0, 10, 10)
res0: geotrellis.raster.TileLayout = TileLayout(0,0,10,10)

scala> TileLayout(-5, -5, 10, 10)
res1: geotrellis.raster.TileLayout = TileLayout(-5,-5,10,10)

where the results are degenerate, nonsensically sized TileLayouts. Changing this to make it impossible would mean a guarantee of sanity in code that expects positively-sized TileLayouts, with the disadvantage of bounds-checking every time one is created.

In the absence of unsigned integer types on the JVM (the proper fix), I suppose the only approach to refusing negative values would be to throw an Exception?