Now everything uses the 'seen' flag in the texture which indicates that the texture was actually used for hardware rendering since the last check.
It's also only the 3 places checking this flag which are responsible for clearing it.
* removed the long defunct EDuke32-based map art system. With our move to named textures this will not work anymore so fixing the existing problems would be pointless.
* moving .def related parsing stuff out of buildtiles.cpp.
* some initial refactoring to make the tile store a static, non-modifyable resource after initial setup.
