Sector.SetAdditiveColor actually called Sector.SetSpecialColor
Add use boolean property, used to determine whether or not to override the sector's additive wall colour with the side's additive colour.
The new specification is more flexible, and allows assigning additive
colors to individual parts of a sector (walls, sprites, flats) and even
individual parts of a side (top, middle, bottom)
Add AdditiveColors arrays to sector_t and side_t::part
Initialize AdditiveColors arrays to 0
Export AdditiveColors to ZScript
Save AdditiveColors in saved game files
Use colors from AdditiveColors arrays when setting the additive color
for the render state
Add code to parse the new UDMF additive color properties
Remove additive color slot from sector color/part enum
Add SetAdditiveColor to sector_t and side_t
Add GetAdditiveColor to side_t
Export new methods and additive color arrays to ZScript
# Conflicts:
# src/hwrenderer/scene/hw_sprites.cpp
# src/hwrenderer/scene/hw_walls.cpp
# src/hwrenderer/scene/hw_weapon.cpp
# Conflicts:
# src/gl/scene/gl_walls_draw.cpp
# src/gl/scene/gl_weapon.cpp
# src/hwrenderer/scene/hw_flats.cpp
# src/r_defs.h
Rename ColorAdd to AddColor
Add AddColor to FRenderState
Tweak SpecialColors array in ZScript to include the additive color
Add uAddColor to the shader compiler
Add uAddColor to the texel
# Conflicts:
# src/gl/shaders/gl_shader.cpp
# src/hwrenderer/scene/hw_flats.cpp
# src/hwrenderer/scene/hw_renderstate.h
# src/hwrenderer/utility/hw_draw2d.cpp
# Conflicts:
# src/gl/scene/gl_flats.cpp
# src/hwrenderer/scene/hw_flats.cpp
