lavc: add ONLY_IF_THREADS_ENABLED where it is missing

Signed-off-by: Paul B Mahol <onemda@gmail.com>
tiff: simplify picking right pixel format

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Add unscaled converter from GBRP > 8bit to RGBx48/64.

Fixes ticket #2633.

Tested-by: Michael Cinquin