...originally introduced with 02abccf9 but
erroneously purged with 9c2f7056.

To make it clear that the second argument is optional.

For example Apache FOP generates RTF output where the \picw and \pich
values are even negative, SvxRTFParser::ReadBmpData() used the same
trick to solve the issue.

With 800806ba matrix decompose
returns the correct angle right away, no need to catch it here.