Merge branch 'OverlayAlpha' into OverlayExtension9

# Conflicts:
#	src/p_pspr.cpp
#	src/r_draw.cpp
#	wadsrc/static/actors/actor.txt
#	wadsrc/static/actors/constants.txt

src/r_data/renderstyle.h

@@ -125,6 +125,9 @@ enum ERenderFlags
 	// Actors only: Ignore sector fade and fade to black. To fade to white,
 	// combine this with STYLEF_InvertOverlay.
 	STYLEF_FadeToBlack = 64,
+
+	// Force alpha.
+	STYLEF_ForceAlpha = 128,
 };
 
 union FRenderStyle