dem_mosaic --priority-blending-length blends where there is no need for it

[oleg-alexandrov]

If there is a DEM that is fully valid everywhere in its domain except for a little hole in the center where data is missing, and one attempts there to blend with another DEM using --priority-blending-length, the blending should happen only there. Yet, apparently, the DEM edge (pixel row row and col are either 0 or image size) is also treated as a place where there is no data because it can't see further pixels and blending is attempted there too. I think this comes from the fact that the weights we use naively decay to zero each time one does not see in all directions from a given pixel, and the true image boundary (as opposed to the image continuing but with no data in those pixels) needs to get special handling.

The testcase is on lunokhod1 in /home/oalexan1/projects/dem_mosaic_blending.

[ScottMcMichael]

I added an option in 4a8c57b that mostly stops border blending but it looks like there is still some unnecessary blending going on.