Made font cache not clutter pixmap with white space glyphs

  - White space glyphs are not loaded into the font pixmap, a dummy
    glyph with no representation is added instead
  - The glyph renderer simply skips dummy glyphs
  - Pixmap load functon no longer needs to clear areas(not that easy
    with OpenGL implementation anyway)
Made X11 font cache completely platform independend
Encapsulated font cache data in a struct
Primitively forced dynamic allocation on font cache

  - Huge potential for simplifications
      -> insert doesn't need cache
      -> create node first, then insert. No need to drag cache along
         recursion
Cleaned font cache code a bit

  - Insertion only needs sub tree root and insertion node
      -> Does not need to drag cache object along the recursion
      -> Create and insert new glyph if required, no search required
         after insertion
  - Sperated tree insertion and tree balance
  - Propperly prefixed tree related functions with "tree_"
Moved font cache to platform independend code