Added (unstable) support for wall textures.
Added (unstable) support for wall textures.

64x64 wall textures can now be loaded by TextureManager::loadIMG().
However, it requires taking away some of the error checking for IMG
files. The loadIMG() code should be improved to make loading wall
textures more dependable. In the future, there should also be a
loadSET() method for loading groups of wall textures.

kcat/OpenTESArena,afritz1/OpenTESArena,kcat/OpenTESArena