[New feature] Register images to union of images extent for mosaic

[gnthibault]

As discussed in the discord, it would be nice to have a feature that would allow to build on the fly, the composited image as a union of the footprint (or extent) of all the previously seen images, in order to enable building mosaic.

Right now, we are using intersection of the footprint(or extent) to build the reconstruction.
To enable this feature, and at the same time, keep computing the average properly on the whole image, we might need to implement a map of image_count, as big as the composite image instead of simple count.

To do so, we might need to use the footprint return value from https://github.com/toros-astro/astroalign/blob/master/astroalign.py#L355