Simple Optimizations for Airplanes and Objs

Just a few basic things:

For airplanes where you don't want to show the PlaneMaker part (because you've rebuilt the plane visually using OBJs):

• In X-Plane 9, set the parts to be invisible - this is faster than drawing them with a transparent texture.
• In X-Plane 8, if the texture is transparent, please downsize it! A 1024x1024 texture that's fully transparent is just a waste of VRAM!

For any version, any object, avoid using ATTR_diffuse_rgb when possible. You can get the same effect by tinting your texture and save unnecessary state thrash.