After noticing that a couple of ERZ's gravity-related gimmicks don't work at all for taller characters*, this branch was started to implement them 'properly' as their own features. Along the way, a couple of gravity-related bugfixes were implemented as well.
Added sector type 1536, Flip Gravity on Jump, which does what it says on the tin (think VVVVVV)
Added an option to linedef type 433 (Enable/Disable Gravity Flip) that will invert the player's current gravity
Uses '[5] Slope Skew' flag in binary
Added a flag to linedef type 1 (Per-Sector Gravity) that will override MF2_OBJECTFLIP when combined with the "Flip in reverse gravity" flag, changing the player's reverse gravity from permanent to temporary
This allows for easier creation of 'walkable ceiling' areas
Uses '[13] Effect 6' flag in binary
Added linedef executor triggers that check the gravity of a player/object
Actions 343-345 in binary, action 343 in UDMF
Bugfixes:
Elemental Shield:
Fixed the stomp's flames having the wrong gravity while using Gravity Boots (Fixes #430 (closed))
Removed MT_SPINFIRE-specific hack in P_GetMobjGravity, and fixed the underlying issue
Included the item dropping changes from !1603 (closed), plus Lach's correction from the comments (Fixes #661 (closed))
Fixed quicksand not behaving properly when in reverse gravity (Fixes #378 (closed))
* This, plus zoom tubes being too small for them, will be changed in an upcoming patch.
