Mgv6 mudflow: Also check for 'ignore' nodes

Mgv6 mudflow: Also check for 'ignore' nodes

Previously, when removing decorations we searched upwards and removed until we
found air or water. However, the node above the decoration can be 'ignore' if
a stacked decoration extends into the volume above the mapchunk. The result
could be a problematic column of air placed in the volume of 'ignore'. The
unnecessary placing of air also slows the function.

Add a check for 'ignore' nodes when removing decorations.