java: fix Mat.toString() for higher dimensions

modules/core/misc/java/src/java/core+Mat.java

@@ -954,8 +954,11 @@ public class Mat {
     // javadoc:Mat::toString()
     @Override
     public String toString() {
-        return "Mat [ " +
-                rows() + "*" + cols() + "*" + CvType.typeToString(type()) +
+        String _dims = (dims() > 0) ? "" : "-1*-1*";
+        for (int i=0; i<dims(); i++) {
+            _dims += size(i) + "*";
+        }
+        return "Mat [ " + _dims + CvType.typeToString(type()) +
                 ", isCont=" + isContinuous() + ", isSubmat=" + isSubmatrix() +
                 ", nativeObj=0x" + Long.toHexString(nativeObj) +
                 ", dataAddr=0x" + Long.toHexString(dataAddr()) +