increasing precission of the quantization parameter

this is needed as the quantization stepsize for each subband is also in this precission and insignificant changes to the wavelet like scaling its coefficients slightly differently would lead to wildly variing PSNR and bitrate
note, a encoder could also simply choose to leave the least significant bits of the quantization parameters zero which would give the exact previous behaviour except a y very tiny number of bits in  the header

Originally committed as revision 4115 to svn://svn.ffmpeg.org/ffmpeg/trunk