Mgv6 mudflow: Remove decoration if 'dirt with grass' below flows away (…

…#5798)

Mudflow of a neighbouring mapchunk extends into a mapchunk's edge, and could
remove 'dirt with grass' from under a decoration, creating unsupported
decorations.

Remove any decoration above if a 'dirt with grass' node is removed by mudflow.

src/mapgen_v6.cpp

@@ -836,13 +836,17 @@ void MapgenV6::flowMud(s16 &mudflow_minpos, s16 &mudflow_maxpos)
					v3s16(-1, 0, 0), // left
				};

				// Check that upper is air or doesn't exist.
				// Cancel dropping if upper keeps it in place
				// Check that upper is walkable. Cancel
				// dropping if upper keeps it in place.
				u32 i3 = i;
				vm->m_area.add_y(em, i3, 1);
				if (vm->m_area.contains(i3) == true &&
						ndef->get(vm->m_data[i3]).walkable)
					continue;
				MapNode *n3 = NULL;

				if (vm->m_area.contains(i3)) {
					n3 = &vm->m_data[i3];
					if (ndef->get(*n3).walkable)
						continue;
				}

				// Drop mud on side
				for (u32 di = 0; di < 4; di++) {
@@ -885,10 +889,18 @@ void MapgenV6::flowMud(s16 &mudflow_minpos, s16 &mudflow_maxpos)
					if (!dropped_to_unknown) {
						*n2 = *n;
						// Set old place to be air (or water)
						if (old_is_water)
						if (old_is_water) {
							*n = MapNode(c_water_source);
						else
						} else {
							*n = MapNode(CONTENT_AIR);
							// Upper (n3) is not walkable or is NULL. If it is
							// not NULL and not air and not water it is a
							// decoration that needs removing, to avoid
							// unsupported decorations.
							if (n3 && n3->getContent() != CONTENT_AIR &&
									n3->getContent() != c_water_source)
								*n3 = MapNode(CONTENT_AIR);
						}
					}

					// Done