avcodec/rawdec: decode 16-bit aligned and packed 'raw' pixel formats where bits_per_coded_sample < 16

The bit packing method is communicted via codec_tag:
    BIT[0]  big-endian packing
    BIT[16] 16-bit little endian packing
    BIT[32] 32-bit little-endian packing
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>