demux: mp4: fix reading vpcC box

The VP Codec ISO Media File Format Binding specification states that the VP codec configuration box (vpcC) extends the FullBox and a FullBox starts with a 1 byte version followed by 3 bytes flags.

Even though we are not interested in the flags, we still have to read them, else all following reads will read from the wrong position, resulting in, among other things, bogus values in the i_codec_init_datasize which is supposed to be always 0 for VP8 and VP9.