swscale: use 15-bit intermediates for 9/10-bit scaling.

[ffmpeg] / tests / ref / vsynth1 / dnxhd_720p_10bit

cb29b6ae4e1562d95f9311991fef98df *./tests/data/vsynth1/dnxhd-720p-10bit.dnxhd
 2293760 ./tests/data/vsynth1/dnxhd-720p-10bit.dnxhd
2f45bb1af7da5dd3dca870ac87237b7d *./tests/data/dnxhd_720p_10bit.vsynth1.out.yuv
stddev:    6.27 PSNR: 32.18 MAXDIFF:   64 bytes:   760320/  7603200