Add R_SplatSlope, function to set slope of splat during rendering

Slope set by R_SplatSlope is only effective if
floorspriteslope or standingslope is not already being
used.

src/r_splats.c

@@ -141,6 +141,7 @@ static void rasterize_segment_tex(INT32 x1, INT32 y1, INT32 x2, INT32 y2, INT32
 void R_DrawFloorSplat(vissprite_t *spr)
 {
 	floorsplat_t splat;
+	pslope_t localslope;
 	mobj_t *mobj = spr->mobj;
 	fixed_t tr_x, tr_y, rot_x, rot_y, rot_z;
 
@@ -258,6 +259,9 @@ void R_DrawFloorSplat(vissprite_t *spr)
 
 		if (standingslope && (renderflags & RF_OBJECTSLOPESPLAT))
 			splat.slope = standingslope;
+
+		if (!splat.slope && R_SplatSlope(mobj, (vector3_t){splat.x, splat.y, splat.z}, &localslope))
+			splat.slope = &localslope;
 	}
 
 	// Translate

src/r_spritefx.cpp

@@ -10,6 +10,7 @@
 
 #include "d_player.h"
 #include "p_tick.h"
+#include "r_splats.h"
 #include "r_things.h"
 
 INT32 R_ThingLightLevel(mobj_t* thing)
@@ -29,3 +30,17 @@ INT32 R_ThingLightLevel(mobj_t* thing)
 
 	return lightlevel;
 }
+
+// Use this function to set the slope of a splat sprite.
+//
+// slope->o, slope->d and slope->zdelta must be set, none of
+// the other fields on pslope_t are used.
+//
+// Return true if you want the slope to be used. The object
+// must have RF_SLOPESPLAT and mobj_t.floorspriteslope must be
+// NULL. (If RF_OBJECTSLOPESPLAT is set, then
+// mobj_t.standingslope must also be NULL.)
+boolean R_SplatSlope(mobj_t* mobj, vector3_t position, pslope_t* slope)
+{
+	return false;
+}

src/r_things.h

@@ -94,6 +94,7 @@ boolean R_ThingIsFullDark (mobj_t *thing);
 boolean R_ThingIsFlashing(mobj_t *thing);
 
 INT32 R_ThingLightLevel(mobj_t *thing);
+boolean R_SplatSlope(mobj_t *thing, vector3_t position, pslope_t *slope);
 
 // --------------
 // MASKED DRAWING