Fixes STJr/SRB2#865

UDMF: Linedef executors

See merge request STJr/SRB2!1625

UDMF: The whole thing

See merge request !1714

# Conflicts:
#	extras/conf/udb/Includes/SRB222_things.cfg

# Conflicts:
#	src/p_mobj.c
#	src/p_spec.c

# Conflicts:
#	src/p_spec.c

# Conflicts:
#	src/p_spec.c

# Conflicts:
#	src/p_inter.c

Fix frame blend flags not working with 3D models

See merge request STJr/SRB2!1778

Fix papersprites being rotated incorrectly in Software when viewing them on the flipped side

See merge request !1770

Add angle field to linedef_t

See merge request STJr/SRB2!1602

Prevent Hangsters from sliding across the ceiling

Closes #787

See merge request STJr/SRB2!1766

Fix Ideya Capture sometimes stealing extra spheres

See merge request STJr/SRB2!1768

Do not reload every texture when adding a wad

See merge request !1725

Drop shadow fake floor/ceiling fix

See merge request STJr/SRB2!1638

Fix Minus & debris' reverse gravity behavior

See merge request STJr/SRB2!1764

Show overall grade emblem requirement of a multi-mare NiGHTS stage in SP pause menu

Closes #71

See merge request STJr/SRB2!1762

Remove drop shadow from badnik explosions, give drop shadows to Metal and flickies

Closes #218

See merge request STJr/SRB2!1761

Make S_IVSP and  S_THUNDERCOIN_SPARK fullbright

See merge request STJr/SRB2!1755

Make P_SpawnGhostMobj ghosts properly copy spritestuff2 (!1257) variables

See merge request STJr/SRB2!1741

Level platter name improvements

See merge request STJr/SRB2!1730

Also clean up redundant "startingspheres" variable