I have implemented smoothing of transitions between surface textures. Now they blend with each other like in "usual" game engines. Compare the next two screens - the before and after modification. The color change is due to blending of the grass level with the tree level.
Transitions in some places are not smooth, and I don't know why. But I think that it is better to have sharp edges somewhere on the planet. I've left some sharp transitions, shown between rock/grass, because the smooth ones look unrealistic. Some more screenshots:
Also, I've updated the code for the mountains, which now creates something that looks like true mountain ranges: