Use threading in dump-frames.rs example

[Architector4]

This significantly speeds up decoding with very little extra code. With the terse documentation, I think it's a good idea to expose that this is something one can do in an example.

Without this patch:

$ time cargo run -- vinny.mp4
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.06s
     Running `/home/architector4/.cache/cargo-builds/cargo-builds/debug/ffmpeg-example vinny.mp4`

real	0m14.942s
user	0m6.685s
sys	0m8.095s

With it:

$ time cargo run -- vinny.mp4
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.06s
     Running `/home/architector4/.cache/cargo-builds/cargo-builds/debug/ffmpeg-example vinny.mp4`

real	0m10.533s
user	0m9.327s
sys	0m8.823s

[Architector4]

oh oops left an extra newline lmao

no biggie i hope?