Fix [#28654] Warp modifier does not support negative strength when Vertex Group is used.

The vg weight was multiplied by org strength (i.e. neg strength was always skiping all verts!), now multiplying it with abs value of strength.

source/blender/modifiers/intern/MOD_warp.c

@@ -232,8 +232,8 @@ static void warpModifier_do(WarpModifierData *wmd, Object *ob,
 				dv = &dvert[i];
 
 				if(dv) {
-					weight = defvert_find_weight(dv, defgrp_index) * wmd->strength;
-					if(weight <= 0.0f)
+					weight = defvert_find_weight(dv, defgrp_index) * strength;
+					if(weight <= 0.0f) /* Should never occure... */
 						continue;
 				}
 			}