* - For GL_TEXTURE_3D, it is the texture's depth at this miplevel. Its
* value, like width and height, varies with miplevel.
* - For other texture types, depth is 1.
* - For GL_TEXTURE_3D, it is the texture's depth at this miplevel. Its
* value, like width and height, varies with miplevel.
* - For other texture types, depth is 1.