We need to handle the backbuffer before we fire the loadend event.
Otherwise listeners that call e.g. mergeNewParams() will cause the
backbuffer removal code to fail, because tile.imgDiv (and hence
this._transitionElement) will be null.
This change introduces a new 'replace' mode for tile transitions: when the
resolution does not change, which happens when mergeNewParams is called,
the tile will be marked with the .olTileReplace class. If this class sets
the tile's imgDiv display to 'none', the backbuffer for the tile will
immediately be removed when the tile is loaded.
The loadend event of an image is fired before the image is rendered. For
standard 256x256 tiles, this does not matter. But for singleTile layers on
large screens, rendering time needs to be considered. So we add a delay
that depends on the tile size. TODO: make the denominator configurable.
We now reuse tile images by maintaining a cache of image elements with a
simplified LRU expiry policy (by order, not by timestamp). The tile queue
is bypassed for images that are available in the cache, so they can be
rendered immediately. And the tile queue itself loads more than just one
image at a time now (2 per layer url).
In addition to relying on removeBackBufferDelay, we can remove the
backbuffer earlier without flicker in an ontransitionend listener on the
last loaded tile.
Calculating pixel positions from origin and grid index causes alignment
issues in the grid. By going back to incremental positioning, we get a
result without blank spaces between tiles again.
Now we also do not use deltas for shiftRow and shiftColumn. Some
refactoring was done so we do not need different calculateGridLayout
methods for layers with top-left and bottom-left tile origin.
TODO: With this commit, ArcGisCache and KaMap layers are broken.
This is because the threshold used for deciding when a column or row is shifted is too far to the bottom right. A tiny fix, but effective. A new test makes sure that we don't shift columns more than necessary when the layer is dragged.
If a layer is configured with serverResolutions, then getServerZoom should return the zoom level as index of the current resolution in the serverResolutions array.
