Added alpha texture map to object exporter

The object exporter does not add the map_d value for the alpha texture map. This is required if you have a transparent texture like glass. For example:

newmtl glass
Kd 0.730570 0.928152 1.000000
illum 2
Ns 50.781250
map_Kd glasswall_glass_color.jpg
map_d glasswall_glass_alpha.jpg