lavfi: remove alphaextract special code

Signed-off-by: Paul B Mahol <onemda@gmail.com>
ffadler: fix duplicate \n on errors

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavu: add FF_CEIL_RSHIFT and use it in various places.
lavfi/overlay: simpler up-rounded w/h computations.
lavfi/opencl: replace SHIFTUP with FF_CEIL_RSHIFT.
Add fate test for solid colour lagarith frames.