Fixed failure to render textures on models in dr/renderer_test.map

- Removed references to the "GDK" ImageLoader module, which no longer exists.
- Introduced the ImageLoaderWx module, which uses a wxImage to load any image
  file that wxWidgets can handle. This replaces home-grown BMP loading code
  that was previously in bmp.{h,cpp}.
- Issue only affected materials that use built-in images such as "_white",
  which are probably rare outside of test maps.