avcodec/sanm: better frame size detection for old codecs

The size of the video frame (FOBJ) of the old codecs (ANIMv0/1/2) can
very reliably be determined:
- ANIMv0/1 (=Rebel Assault 1) uses a 384x242 internal buffer for
  everything.  The codec parameters only describe the size and offset
  of the specific FOBJ on that buffer.
- ANIMv2 titles usually use one of the fullscreen codecs (37/47/48)
  as first FOBJ, and their dimensions can generally be trusted.
- RA2 uses 424x260 as internal buffer, use that if encountered:
  08PLAY.SAN does not use codec37 so we need to guess using the
  codec coordinates.
- ignore sizes smaller than 2x2 or larger than 800x600.
- some game videos have an initial fobj with either 1x1 or -1x-1
  pixels in size, ignore them with a warning (Full Throttle
  and the Rebel Assault 2 xxRETRY.SAN videos).

Once a known/valid dimension set has been discovered, use it and
don't change it for subsequent FOBJs, rather clamp the large frame
to the determined dimensions.

Tested with RA1, RA2, Full Throttle, Dig, Outlaws, SotE and MotS
videos.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>

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