Further cleanups to texture caching, from bug 523:

Further cleanups to texture caching, from bug 523:

- Texture::filename was used for only one purpose, so we might as well
  make that clear.

- Set the filename in just one place: before a texture is added to the
  cache.  This is intended to be obviously correct.

- Other minor adjustments.

Thanks to Matt McCutchen for this patch.

git-svn-id: http://supertux.lethargik.org/svn/supertux/trunk/supertux@6294 837edb03-e0f3-0310-88ca-d4d4e8b29345