Add example for comparing AV1 with and without grain synthesis

[fabianwimberger]

Adds a README example showing how to use --right-decoder libdav1d:export_side_data=film_grain to compare an AV1 video against itself with film grain synthesis disabled on the right side.

This is useful for evaluating the visual impact of AV1 film grain synthesis but requires libdav1d to be available.

[jonfryd]

This is a nice addition.

The description (“with and without film grain synthesis applied”) is clear, but the actual argument libdav1d:export_side_data=film_grain is a bit non-obvious, since it looks like a metadata/export option rather than something that changes decoding behavior. Would it make sense to add a short inline note that, for libdav1d, this also disables film grain synthesis by default? That connection isn't immediately clear unless you're familiar with the FFmpeg details.

Also, minor stylistic suggestion: I think it would improve readability to place decoder options before the input files for consistency. Something like:

video-compare --right-decoder libdav1d:export_side_data=film_grain input_av1.mkv __

But overall, great addition.

[fabianwimberger]

Thanks for the review! I've added a short note clarifying that `export_side_data=film_grain` disables grain synthesis during decoding, and moved the decoder option before the input files for consistency.

Would be glad to see others check their av1 grain synth results with this project.