vf_fade: Make sure to not miss the last lines of a frame

When slice_h is rounded up due to chroma subsampling, there's
a risk that jobnr * slice_h exceeds frame->height.

Prior to a638e918, this wasn't an issue for the last slice
of a frame, since slice_end was set to frame->height for the last
slice.

a638e918 tried to fix the case where other slices than the
last one would exceed frame->height (which can happen where the
number of slices/threads is very large compared to the frame
height).

However, the fix in a638e918 instead broke other cases,
where slice_h * nb_threads < frame->height. Therefore, make
sure the last slice always ends at frame->height.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>