NO MORE HALF MEASURES, CLUSTER

Created by: DrCelt

this is my largest PR to date in terms of files changed, and sum of lines changed. why did I agree to do planes

ADDS:

• layers are now defines (this is a big deal)
• added some documentation to _planes+layers to help people I guess

TESTING THE FIX FOR: (not having an X doesn't mean I haven't fixed it, just means I haven't yet tested without a doubt whether or not it IS fixed)

• DISPOSAL PIPES VERSUS AIR PIPES - fixes #10756 (closed)
• CABLES VERSUS TERMINAL
• PLAYER VERSUS ITEMS IN HAND - fixes #10804 (closed)
• POINTER VERSUS OTHER THINGS - fixes #10694 (closed)
• FORCEFIELDS INVISIBLE (you guys did realise this isn't a planes issue right)

NEW BUGS FOUND:

(if box is checked, the bug has been fixed.)

• /obj/screen/inventory used the wrong layer, layer = 19.
• all hud objects appear less responsive (probably to do with using my laptop trackpad, actually.)
• if you slip on the clown's PDA, you will appear underneath it, rather than on top. - will need to re-evaluate the object plane. (never mind, I just gave blankets their own plane and bumped up lying humans. should work fine)
• damage overlays are above the HUD, rather than below. Should give them their own plane, I think. (done)
• cables are on the wrong plane.
• pipes are on the wrong plane.
• Privacy shutters are on the wrong plane.
• Forgot to put humans on the lying human plane when they are lying.
• Forgot to put humans on the human plane when they are not.
• Status Displays are on the wrong plane
• MoMMIs appear blind when hiding
• Door layering is wacky.
• MoMMIs cannot unhide.
• density of poddoors New() wrong way around
• side windows appeared above poddoors
• UNFIXABLE: North facing chairs appear above objects that they shouldn't. (This is unfixable. - You cannot have them appear above humans and below objects which appear below humans. - I imagine that this was already a bug.)
• Cryocells were on the wrong layer.
• Catwalks were on the wrong plane.
• Showers & Curtains behaved oddly when combined. Fixed.
• signs did not appear above windows. They do now.
• Monkies did not layer correctly when lying down.
• Accidentally pushed the wrong map.
• Wrong window was on top of the other.
• Alien nests layered incorrectly.
• Molten items did not layer correctly.

100% TESTED:

• SECURITY CAMERAS AND LIGHTBULBS APPEAR ABOVE HUMANS
• NOTHING APPEARS ABSOLUTELY FUBAR SO FAR
• Comfy Chairs, Secways, Janicarts & Chairs appear normal
• Turrets seem normal
• Labcoats work
• Bags work
• Pockets work
• MoMMIs hats still work, their eyes still appear above darkness, and dropping from them layers correctly.
• The Holomap renders appropriately. This should be turned into a plane.
• Mesons still work
• In-hand cyborgs work as intended.
• oh wow the engineering borg sprite has spooky eyes
• People can still hide behind plants
• Blobs seem to still layer correctly.
• corgi and carp hearts layer correctly
• cages layer correctly
• Sorry, JustSumGuy, this definitely does not fix cameras.
• cobwebs layer correctly.
• liquid fuel layers correctly

TODO:

• will need to rethink the OBJ_PLANE, it's too big and all encompassing, see the clown-PDA bug (never mind, I just gave blankets their own plane and bumped up lying humans. should work fine)
• Expand the tested list...?
• Doors handled layers really weirdly, DOOR_MOD and such. Make it use var/open_layer & var/closed_layer instead.
• Read through my changes and try and catch out mistakes, such as putting OBJ_PLANE where I meant OBJ_LAYER. I'm almost certain I've done this somewhere.
• I don't trust turrets, they had really funky layers when I was looking at them, let's take a look if I did it correctly. (Seem okay.)
• Look at every single department on Box station and check if anything is amiss.
• Do the same with Defficiency.
• fix the code issues I pointed out

This was all done by hand, I don't know fancy replace code