SE5: Limitations on modding unit graphics

[Captain Kwok]

I'm actually hopeful that the sprites can actually be extended to all ships in the VehicleSizes.txt file as an additional line in that file with a picture reference to a bmp with the "basic textures". This would allow for lower-end systems to run larger combats in a low-end mode. This would eliminate the need for that line in [Empires]_AI_Main.txt and add some additional flexibility for users.