alien weeds performance and layer fixes

recently i fixed a couple of issues with /vg/'s alien weeds and nodes, 
and i see some applying to Bay12 too.

one of those issues was the order of checks in weeds/Life().
it should cancel the weed's search for tiles to expand to without starting
the direction loop first, saving quite a few proc calls.

another issue is alien weeds having the default OBJ_LAYER layer 3 which makes them grow
over all kinds of items and objects which makes sense i guess fluffwise but
is really annoying ingame for both crew and aliens.
( and this coming from the host of Alium Deathtrap 13 ;) )
nodes stay at layer 3 to appear properly over things like AI-holopads.

a really tiny improvement is setting linked_node after the space-turf check in weeds/New().

link to the /vg/ commit : https://github.com/d3athrow/vgstation13/commit/4dcb434f7258fe2acd266a1709ab2022389eb845