Fix DPX decoder

Rewrite the DPX decoder to work with provided sample DPXs at
http://samples.ffmpeg.org/image-samples/dpx_samples.zip

The decoder could only decode 8 and 10 bit without alpha correctly,
failing or even crashing at other flavors. This patch aims to fix
these issues, properly decoding all variants of DPX provided in the
referenced DPX sample zip. For 10 and 12 bit, the alpha channel
is ignored, but decoding is still possible.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
