Zoomify and backBuffer

Zoomify bottom and rightmost images might have size different from standardTileSize. backBuffer creation and display should take this into account.
I think applyBackBuffer should user this.tileSize to determine tile top/left position because all tiles avec same dimensions. So _w and _h should only be used to adjust tile display size.

I modified createBackBuffer but I'm not sure it won't break backBuffer for other layers using Grid.

Zoomify.getImageSize() should user gridResolution as it can be called to display images for backBuffer or base layer.