Bump mapped models

Models can use shader_model_extended, a model shader with everything a regular shader_model has plus:

• Normal maps, with up to two detail normal maps, so surfaces show bumps and grooves that react to the lights around the object.
• Specular highlights from the lights around the object, tinted per pixel by a specular color map.
• A second detail map, masked by the multipurpose map like the first one.

Maps have to use the new tag to get them; models with regular shader_model tags look as they always did. Map creators can find the fields in Shader model extended.

balltze_set_model_shader_extension 0 draws those tags as plain model shaders, to compare the two. The setting is not saved.