The new class TileNode represents a node in the tree structure
whereas Tile is just a data structure to store the pixmap and tile rect
and is used outside tiles manager.
The miss counter was taken away. Now the algorithm relies on the
distance between the tiles and the viewport.
Also the visibleRect() and setVisibleRect() methods were removed from
TilesManager since we now pass this information to
TilesManager::cleanupPixmapMemory()
Some requests may take a while to process in a highly zoomed region.
Since those requests are not yet finished, other requests to the same
region are likely to be made (specially if the user is scrolling fast)
and the generator will have to render the same region repeatedly.
Also when changing zoom rapidly some pixmaps can arrive at the tiles
manager when another request has already been posted. This pixmap is not
necessary anymore and should be discarded (the tiles manager will get a
new pixmap anyway).
The splitting was only executed after the pixmap arrived the tiles
manager. That was bad and likely to lead to an unnecessary rendering in
the case of a big tile that would be split after all.
This also fixes a bug where some tiles weren't updated.
