Color to alpha is applied to all layers indifferently

[guisalmon]

Code :

import logging
from landez import MBTilesBuilder
from landez import TilesManager
from landez.filters import ColorToAlpha


logging.basicConfig(level=logging.DEBUG)

mb = MBTilesBuilder(mbtiles_file="/home/guillaume/Tiles/Merge/tiles.mbtiles",
                    cache=True, filepath="/home/guillaume/Tiles/Merge/blended.mbtiles")
overlay = TilesManager(mbtiles_file="/home/guillaume/Tiles/Merge/tiles_kml_v3.mbtiles")
overlay.add_filter(ColorToAlpha('#ffffff'))
mb.add_layer(overlay)
mb.add_coverage(bbox=(4.9919,42.8865,5.6985,43.2997),
                zoomlevels=range(10,18))
mb.run()

This is supposed to blend osm maps (tiles.mbtiles) and a layer containing colored areas (tiles_kml_v3.mbtiles) with #ffffff to alpha in the second layer. The fact is, white is sent to alpha in both layers but only in the areas defined in the second layer. This is the result :

(Yes, with the white background on github, it doesn't change anything, it is more evident on a non-plain background, on GIMP for instance)