Make particles face in view direction (not toward viewer)

Make RT_SPRITE entities whose shaders were registered with RSF_SPRITE
face opposite the view direction, instead of toward the viewer. This
prevents pathological cases where the normal of a closeby sprite
is oriented nearly orthogonally to the view direction, making its 2-D nature
obvious and ugly. I found that to happen with the alien evolve acid
particle system if I walked backward right after evolving from Dretch to
Mantis.

RT_SPRITE entities without the RSF_SPRITE shader flag continue to face
toward the viewer. Light flares go in this category.

src/engine/renderer/tr_local.h

@@ -1293,6 +1293,9 @@ enum class realtimeLightingRenderer_t { LEGACY, TILED };
 
 		bool        interactLight; // this shader can interact with light shaders
 
+		// For RT_SPRITE, face opposing the view direction rather than the viewer
+		bool entitySpriteFaceViewDirection;
+
 		int		autoSpriteMode;
 
 		uint8_t         numDeforms;

src/engine/renderer/tr_shader.cpp

@@ -6363,6 +6363,11 @@ shader_t       *R_FindShader( const char *name, shaderType_t type,
 		implicitCullType = CT_FRONT_SIDED;
 	}
 
+	if ( flags & RSF_SPRITE )
+	{
+		shader.entitySpriteFaceViewDirection = true;
+	}
+
 	// attempt to define shader from an explicit parameter file
 	shaderText = FindShaderInShaderText( strippedName );
 

src/engine/renderer/tr_surface.cpp

@@ -596,20 +596,35 @@ static void Tess_SurfaceSprite()
 		           tess.surfaceShader->name );
 	}
 
-	VectorSubtract( backEnd.currentEntity->e.origin, backEnd.viewParms.pvsOrigin, delta );
+	VectorSubtract( backEnd.currentEntity->e.origin, backEnd.viewParms.orientation.origin, delta );
 
 	if( VectorNormalize( delta ) < NORMAL_EPSILON )
 		return;
 
-	CrossProduct( backEnd.viewParms.orientation.axis[ 2 ], delta, left );
+	vec3_t forward;
+	if ( tess.surfaceShader->entitySpriteFaceViewDirection )
+	{
+		// Face opposite to view direction, triggered by RSF_SPRITE.
+		// Good for particles that may appear very close to the viewer and thus have extreme
+		// difference between the view direction and the direction to the viewer, so
+		// as to avoid cases where they appear obviously planar
+		VectorCopy( backEnd.viewParms.orientation.axis[ 0 ], forward );
+	}
+	else
+	{
+		// Face toward viewer. Used by light flares
+		VectorCopy( delta, forward );
+	}
+
+	CrossProduct( backEnd.viewParms.orientation.axis[ 2 ], forward, left );
 
 	if( VectorNormalize( left ) < NORMAL_EPSILON )
 		VectorSet( left, 1, 0, 0 );
 
 	if( backEnd.currentEntity->e.rotation != 0 )
-		RotatePointAroundVector( left, delta, left, backEnd.currentEntity->e.rotation );
+		RotatePointAroundVector( left, forward, left, backEnd.currentEntity->e.rotation );
 
-	CrossProduct( delta, left, up );
+	CrossProduct( forward, left, up );
 
 	VectorScale( left, radius, left );
 	VectorScale( up, radius, up );