Implement simple visualization of object blockmaps

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Visualization in Block mode is very simple - just rectangular blocks, from base height to topmost height. Visualization in Terrain mode is a more interesting - top mesh like a terrain, bottom mesh - like a flipped terrain, sides - like a Block mode.

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Implemented Block mode visualization

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Implemented funny mesh generation for objects with blockmap ("map objects"). I want to use map objects to enrich main terrain with overhangs.

Todo: more accurate align with main heightmap. Some blocks of map object need to be marked as coincide blocks for seamless transition from/to main heightmap.

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The next step - good-looking blending of main and object heightmaps

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Ok, now map object snaps for main map blocks. Still need calculate common normals and resolve z-fight (need to discard occluded main map blocks from meshing)