avcodecmap: Add MxPEG codec ID
The MXPEG AVCodecId was missing from the libav mapping list, so the caps were being set to unknown/unknown, meaning avdec_mxpeg wasn't working. This fixes that
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The MXPEG AVCodecId was missing from the libav mapping list, so the caps were being set to unknown/unknown, meaning avdec_mxpeg wasn't working. This fixes that