avcodec/prores_ks: Fix luma quantization if q >= MAX_STORED_Q

avcodec/prores_ks: Fix luma quantization if q >= MAX_STORED_Q

The problem occurs in slice quant estimation and slice encoding:

If the slice quant is larger than  MAX_STORED_Q we don't use pre-calculated
quant matrices, but generate a new one, but both qmat and qmat_chroma both
point to the same table, so the luma table ends up having chroma table
values.

Add custom_chroma_q the same way as custom_q.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>