Key the painted checkerboard out of the provided gem art via an
edge-connected flood fill (8-conn, light+desaturated predicate) so the
gems' interior specular highlights are preserved, then slice the 6 gems
by alpha bbox, premultiply-resize each to fit 108x108, and center one
per 128x128 transparent cell in index order (red, orange, yellow,
green, blue, purple).
