Test that we correctly assign mask IFDs to Overview for COGs with internal mask

[kylebarron]

GeoTIFFs can have internal validity masks.

Tasks:

• Create an image in geotiff-test-data that has internal masks
• Validate that geotiff.overviews correctly assigns internal masks. You should be able to look at the _mask_ifd attribute on Overview.

[kylebarron]

I think this is closed with #36 and #22