When a GeoTIFF doesn't have a nodata value, and we use a VRT, GDAL fills in 0s for pixels outside the bounds. However, read(masked=True) doesn't mask these 0s, so they're left in as valid data.
Using add_alpha in the VRT, we get an extra explicit alpha band to use as the mask.
When a GeoTIFF doesn't have a nodata value, and we use a VRT, GDAL fills in 0s for pixels outside the bounds. However,
read(masked=True)doesn't mask these 0s, so they're left in as valid data.Using
add_alphain the VRT, we get an extra explicit alpha band to use as the mask.Closes #217.