Improve mosaic management when requesting very small area where no granule data is available

When requesting a very small area (as an instance a tiny vertical stripe having width = 1) on the edge of a mosaic, in case there isn't any granule data to be loaded in that area, the underlying computations may result in defining a raster bound having width (for that example) = 0. Therefore, the JAI "Constant" operation involved in producing the background to fill the requested area may throw an IllegalArgumentException (being the width = 0). In that case, forcing the width of the background stripe to be at least 1 pixel wide, will fix that behaviour.