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.

[sgillies]

@danshapero I'm sorry we've overlooked this for so long. The project has degraded and we no longer have a CI configured to test your changes. Combine that with a lack of long-term plans for this plugin and I'm hesitant to click merge. I need to discuss the future of this project with my team. If we're going to keep maintaining this plugin, a project infrastructure refresh will be followed immediately by a merge of this PR.