lavfi: make ff_framequeue_skip_samples() more useful.

lavfi: make ff_framequeue_skip_samples() more useful.

Instead of just updating statistics and leaving the work to the
call site, have it actually do the work.

Also: skip the samples by updating the frame data pointers
instead of moving the samples. More efficient and avoid writing
into shared frames.
Found-By: Muhammad Faiz <mfcc64@gmail.com>