Texture Paint: prevent normal angle masking duplicating backface culling.

If backface culling is off, the user obviously wants to paint on
back faces, so the normal angle cutoff designed to prevent painting
at glancing angles shouldn't do the culling as a side effect.

source/blender/editors/sculpt_paint/paint_image_proj.c

@@ -1461,6 +1461,11 @@ static float project_paint_uvpixel_mask(
 			angle_cos = dot_v3v3(viewDirPersp, no);
 		}
 
+		/* If backface culling is disabled, allow painting on back faces. */
+		if (!ps->do_backfacecull) {
+			angle_cos = fabsf(angle_cos);
+		}
+
 		if (angle_cos <= ps->normal_angle__cos) {
 			return 0.0f; /* outsize the normal limit*/
 		}