Optimise ToI420 conversion by using sync.Pool

[EmrysMyrddin]

Since #418, the I420 conversion have been slow down because 2 new buffers are allocated for each frame converted.

The idea is to use a sync.Pool to minimise the overhead of creating new []byte:

• we get a []byte with Pool.Get,
• we check if it's size feats the image to convert, growing it if necessary
• use it for the conversion
• return the Image + a release function which calls the Pool.Put function with the used []byte buffers.

This way we should limit the garbage collector pressure and the time needed to fill the []byte with 0 values at creation.

[neversi]

Could I have a try to resolve this issue?

[neversi]

Is there need to add sync.Pool? Whenever there is the call to ReaderFunc, there is also mutex Lock, so here we could use closure instead as there is no concurrent access to the buffers.

[neversi]

Can we close the issue?

[at-wat]

done by https://github.com/pion/mediadevices/pull/473