avfilter/showcqt: remove yuv offset

this makes draw_bar faster
slightly different result with old version

check result (with ~3 minutes audio file):
old:
    real    0m49.611s
    user    0m49.260s
    sys     0m0.073s
new:
    real    0m47.606s
    user    0m47.378s
    sys     0m0.068s
PSNR between old and new:
yuv444p PSNR
    y:109.519298 u:107.506485 v:104.746878
    average:106.816074 min:99.167305 max:inf
yuv422p PSNR
    y:109.519298 u:108.025801 v:104.489734
    average:107.279817 min:98.007467 max:inf
yuv420p PSNR
    y:109.519298 u:108.363875 v:105.290200
    average:108.261511 min:97.461812 max:inf

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>