avfilter/dctdnoiz: make color [de]correlation less clumsy

This has no impact on overall performance, since the block DCT taking
most of the time anyway.
avfilter/dctdnoiz: remove forward declarations after previous commit
avfilter/dctdnoiz: use 32-bit (float) operations instead of 64 (double) for DCTs

This makes the code about 1.5x faster without any noticeable difference
in the output.
avformat/matroskadec: Check avpriv_new_chapter() for failure

Fixes null pointer dereference

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