Decoration API: Add flag for placement on liquid surface

[paramat]

Add findLiquidSurface() function to mapgen.cpp
Update lua_api.txt

Currently a mess is made if water level is changed in mgv5/mgv7. Water lily and papyrus schematics appear at the wrong altitude, the water node placed by the waterlily schematic causes floods. This reminded me to add the long-overdue option of placing decorations on a liquid surface.

The new findLiquidSurface() function searches downwards through a mapchunk column and returns the level of the first liquid node found, if ground is found first the function returns -31000 disabling the decoration placement, this is to prevent decorations being placed under overhangs and in dark places.

Placing waterlilies this way does not allow random rotation, so a follow-up commit could add optional random rotation to simple decorations.

Keeping the waterlilies in shallow water will be another issue but can be solved using biome definitions (making swamp biomes shallower). Anyway it's not absolutely certain we will stop using schematics for waterlilies.

Works for schematic decorations too:

[paramat]

Updated due to decorations being found under overhangs.

The new findLiquidSurface() function searches downwards through a mapchunk column and returns the level of the first liquid node found, if ground is found first the function returns -31000 disabling the decoration placement, this is to prevent decorations being placed under overhangs and in dark places, see screenshot of test above.

[paramat]

Approved http://irc.minetest.ru/minetest-dev/2015-10-23#i_4429842
59fa117