Compares value1 and value2. If value1 and value2 cannot be
compared, the function returns GST_VALUE_UNORDERED. Otherwise,
if value1 is greater than value2, GST_VALUE_GREATER_THAN is returned.
If value1 is less than value2, GST_VALUE_LESS_THAN is returned.
If the values are equal, GST_VALUE_EQUAL is returned.
Compares value1 and value2. If value1 and value2 cannot be compared, the function returns GST_VALUE_UNORDERED. Otherwise, if value1 is greater than value2, GST_VALUE_GREATER_THAN is returned. If value1 is less than value2, GST_VALUE_LESS_THAN is returned. If the values are equal, GST_VALUE_EQUAL is returned.