libavfilter/vf_owdenoise.c: skip processing when strength is 0

It is practical to de-noise only on luma while keeping chroma unchanged.

However, libavfilter/vf_owdenoise.c always do the Wavelet transform/retransform on all 3 channels without check whether chroma_strength is 0.

Thus I make this patch. De-noise on Y only for yuv420 is now 1.5 times faster.

Signed-off-by: Yuuki Galaxy <galaxy001@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>