message: Fix global variable assignment in JS test suite
  In test_imgproc.js, the test_filter suite's last test assigns a variable
  to `size` without declaring it with `let`, polluting the global scope.
  This commit adds `let` to the statement, so that the variable is scoped
  to the test block.
diff: @@ -948,7 +948,7 @@ QUnit.test('test_filter', function(assert) {
  cv.rotate(src, dst, cv.ROTATE_90_CLOCKWISE);
  - size = dst.size();
  + let size = dst.size();
  assert.equal(size.height, 2, "ROTATE_HEIGHT");
  assert.equal(size.width, 3, "ROTATE_WIGTH");
  