Allows to reset notexture, to set a high-contrast color to find and
fix the areas and to set to a useful default texture.
As useful textures, in addition to rock.jpg and concrete.jpg, the menu
lists the default wall, floor and ceiling textures of the current map,
and also the last watched texture from the texture menu.
Also adds a few checks to detect unconverted map config files and misuse
of the new bright highlight textures.
ac-stef