Allow mesh and nodeboxes to wave like plants or leaves. (#3497)

Allow mesh and nodeboxes to wave like plants or leaves. (#3497)

We introduce a new value for "waving" - 2:

0 - waving disabled
1 - wave like a plant
2 - wave like a leave

Plantlike nodes will only allow waving = 1, but for leaves we will
permit both 1 and 2 since current minetest_game sets it to 1 for
all leaves. This makes it somewhat backwards compatible.

For mesh and nodebox, values 1 and 2 are both valid, and the node
can wave in both fashions as desired.

I've tested this with the crops:corn plants, which are mesh nodes,
and the results are really good. The code change is trivial as
well, so I've opted to document the waving parameter in lua_api.txt
because it was missing from there.

Nodeboxes likely will not wave properly unless waving = 2. However
it's possible that waving=1 may be desired by some mod developers
for geometries I have not tried, so the code will not prohibit
either value for mesh and nodebox drawtypes.

Add lua_api.txt documentation for this feature and document both
the existing functionality and the expansion to mesh and nodebox
drawtypes.