function _sAssign(sVal, iVal) {
            if (sVal > 0 && sVal < iVal) {
              return sVal;
            } else {
              return iVal;
            }
          }
Validates clipping params. Per drawImage spec sWidth and sHight cannot be
negative or greater than image intrinsic width and height
@private
@param {Number} sVal
@param {Number} iVal
@returns {Number}
@private