fix (texture) image texture property

Fix to use the image texture as the texture for the model. The image texture can be loaded and edited with the C3 image editor.

Only a single texture for the 3DObject can be currently used. It is advised to use square textures, because the C3 spritesheet may rotate non square textures.

The same image texture applies to all instances of the object that have the image texture property checked, even if they additionally have the instance model property set.