Blender Internal: Fix regression in point density texture

The issue was introduced by original Cycles point density support commit,
it lead to a constant density of 1 for object verticies point density source.

source/blender/render/intern/source/pointdensity.c

@@ -597,6 +597,9 @@ int pointdensitytex(Tex *tex, const float texvec[3], TexResult *texres)
 
 	BRICONT;
 
+	if (pd->color_source == TEX_PD_COLOR_CONSTANT)
+		return retval;
+
 	retval |= pointdensity_color(pd, texres, age, vec);
 	BRICONTRGB;