Fix window padding function for changes to API of rasterio-1.0

[danshapero]

I tried using rio-pansharpen from the command line recently and ran into an error because the Window class no longer supports indexing. I think that class was changed from rasterio-0.36 to 1.0. In any case, this patch changes the window padding function in rio-pansharpen to work with the new rasterio API.