This patch adds slice threading support to the blackframe filter.
It implements the standard Map-Reduce pattern: each thread processes a
vertical slice of the frame, and the main thread sums the results.

Benchmarks:
Input: 4K Video (3840x2160, YUV420P), 250 frames, 100% black.
Command: ./ffmpeg -f lavfi -i color=c=black:s=3840x2160:d=10 -vf blackframe
-benchmark -f null -

Results (on 4-core system):
- Single-threaded: ~0.259s (real)
- Multi-threaded:  ~0.077s (real)
- Speedup:         ~3.36x

Signed-off-by: Raja Rathour <imraja729@gmail.com>