Add support for "excess granule removal" in image mosaic

Some mosaics have a large number of overlapping images associated with some filtering and z-ordering criteria (e.g., freshest on top, color on top, highest resolution on top).

For those cases we don't want to process up to the mosaic point all the images available in the area, but only those that can contribute pixels to the output, basing the decision on their mask. This is known as "excess granule removal"