LayerListBuilder: don't emit useless PaintColrLayers of length=1

We already filter len(layers)==1 out upfront, but these can sneak in after the layer reuse code when input contains two base glyphs that refer to the same list of layers. This ensure that duplicate base glyphs get the same PaintColrLayers, rather than a wrapper PaintColrLayers of length=1 pointing to the underlying shared PaintColrLayer...

Lib/fontTools/colorLib/builder.py

@@ -551,6 +551,10 @@ def listToColrLayers(layer):
 
         layers = [listToColrLayers(l) for l in layers]
 
+        # No reason to have a colr layers with just one entry
+        if len(layers) == 1:
+            return layers[0], {}
+
         paint = ot.Paint()
         paint.Format = int(ot.PaintFormat.PaintColrLayers)
         paint.NumLayers = len(layers)