Gravity flip bugfixes & additions

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.

Test exe: srb2win_gravityflipping.exe - Updated 12/09/2022 at 11:30 CET

Test map: gravtest.wad

Additions:

• 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.