Saner RV1/2 initialisation and RV2 header parsing.

Old version was based on lots of guesswork which had different hacks based on
32-bit version field instead of splitting it into major/minor/micro version.
RV2 picture header parser also had a few places where it deviated from binary
decoder.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
