Use pygfx material.map directly for cmap and cmap_transform #960

See: pygfx/pygfx#1246, #953 (comment)

Related: #629

I think this will mean that we can then use the same map (cmap in fpl) and mapcoords (cmap_transform in fpl) for a histogram LUT tool and/or colorbar that's used as a legend! So it'll make that usecase easier.