Adding boolean attribute "transparent" for files with transparency.

http://stackoverflow.com/questions/18444939/define-transparent-material-in-json-model-format-3-1

utils/converters/obj/convert_obj_three.py

@@ -399,6 +399,7 @@ def parse_mtl(fname):
             # Transparency
             # Tr 0.9 or d 0.9
             if (chunks[0] == "Tr" or chunks[0] == "d") and len(chunks) == 2:
+                materials[identifier]["transparent"] = True
                 if TRANSPARENCY == "invert":
                     materials[identifier]["transparency"] = 1.0 - float(chunks[1])
                 else: