Rendering with or without grid is now possible.
Showing the surrounding landscapes in the main canvas.
Fixed the map creation process, so that there are five different
heights.
A nice effect: A new height difference between layers makes the playing
field more "readable" (3/4 of the tile size).
Refactoring, in oder to deal with a symmetry math problem.
Half way there, I guess.

There is a MirrorModeHelper class now. It helps with the clutter in the
forEachPixelToUpdate method.
There were two versions of the setLowestValue method.
Activated and fixed axis symmetry.
I now have an implementation that supports point symmetry.
The line that is used to cut the map into 2 parts can only be one of
those that are used for axis symmetry.

That limits the variations a little, but it helps with debugging.
I might still try to fix the problem but now I do not have to.
The current implementation is pretty readable and it shows the problems
which are still open on the map.
Removed log message and changed color to show the unsupported and
undefined values differently.
Code deals better with seams (not perfect).
Decided to use the horizontal axis as the mirror line for point
symmetry. The created map is the same in any event, just which points
are rendered when never changes now.
Clicking the canvas now reinitializes the engine and recreates / redraws
the map.