Schematics can already be placed with a param2 value, but not
simple 1-node plant decorations of the simple type.
This adds a `param2` field to the simple deco type that is
checked to be between 0 and 255, and put to the placed node
at mapgen.
This can be used to put a degrotate value in, or e.g. a fill
value for leveltype nodes, or a place_param2 value at mapgen
placement, or vary the shape of meshoptions plantlike drawtype.
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