layer.setTrackMatte took one layer. It now also takes `layers`, set in
one undo step; `layer` and the first selected layer still work as
before. `kind: "preserve"` keeps each layer's own matte mode (Alpha for
a layer without one), so giving several layers a new matte doesn't turn
a mix of Alpha and Luma mattes into one mode; without `kind` the mode
is Alpha, as before. A layer can't be its own matte and an empty list
is refused, before anything changes.
Test: several_layers_take_one_matte_and_keep_their_modes.
tylerjenningsw
f51b4b7e1b