Fix surfaces with single color for all faces/vertices

[adhusch]

Fix surfaces with single color for all faces/vertices that still have indexed color CData set which however is not used by Matlab due to specified single solid color. (Example: create streamtube, set FaceColor for streamtube object, Matlab plots streamtube (correctly) in the set color but fig2idtf still generates an index color stream tube cycling through the complete colormap without this fix)