Support texture flipping for signed versions of RGTC 1 and 2 (#1629)

jme3-core/src/plugins/java/com/jme3/texture/plugins/DXTFlipper.java

@@ -220,9 +220,11 @@ public class DXTFlipper {
                 type = 3;
                 break;
             case RGTC2:
+            case SIGNED_RGTC2:
                 type = 4;
                 break;                
             case RGTC1:
+            case SIGNED_RGTC1:
                 type = 5;
                 break;
             default: