Corrected the warning message that showed up in issue #1140.  We are saving SerializableTile instances, not PNG.
* Dropped ZoomInTileLooper and ZoomOutTileLooper entirely as they no longer do anything.
* Added an invalidate() call on MapView::setZoom so that any zoom
  change forces a viewport redraw

Note that this will snap back your zoom level if you haven't zoomed in
by a full level.  This behaviour was less obvious before the patch,
but is happened before as well.  It just happened in 2 steps.  Pinch
zoom with an edge that is not redraw, then on the next redraw, the
zoom level would snap back.
Merge pull request #1143 from crankycoder/features/1142-fix-partial-zoom

1142 - fix partial zoom