给所有选择的图层进行预合成
var proj=app.project;
var myComp= proj.activeItem;
var myLayers=myComp.selectedLayers;
app.beginUndoGroup(Undo 批量预合成);
for(var a=0;a<myLayers.length;a++){
myComp.layers.precompose([myLayers[a].index], myLayers[a].name+_预合成, false)
}
app.endUndoGroup()