qt-faststart: speedup

qt-faststart is terribly slow when the input file and the output file
are on a slow disk like a SD card. By increasing the copy_buffer from
1K to 32M I decreased the processing time on a sample file from
1600 seconds to 4 seconds. The timing difference is during 'copying
rest of file'.

S:\SD_VIDEO\PRG001>e:\utils\qt-faststart 00005.mp4 5.mp4
ftyp          0 32
free         32 8
mdat         40 13744391
moov   13744431 141848
 patching stco atom...
 patching stco atom...
 writing ftyp atom...
 writing moov atom...
 copying rest of file...

Execution time: 1576.259 s

S:\SD_VIDEO\PRG001>s:\utils\qt-faststart 00005.mp4 5.mp4
ftyp          0 32
free         32 8
mdat         40 13744391
moov   13744431 141848
 patching stco atom...
 patching stco atom...
 writing ftyp atom...
 writing moov atom...
 copying rest of file...

Execution time: 3.846 s

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