Add R_TextureNameForNum and R_CheckTextureNameForNum
The lack of a function like this is very strange considering that sidedefs use numbers for their textures instead of full names. if you need those names you didn't have any way to get it. This merge request aims to fix that.
Adds both R_TextureNameForNum and its Check variant
the difference is that R_TextureNameForNum returns REDWALL if no texture is found while R_CheckTextureNameForNum returns - or the No Texture marker
The included script uses R_TextureNameForNum to get the names of every sidedef texture and prints all non REDWALL values on PlayerSpawn