Allow providing output pixel format for decoder

[gBillal]

We want the ability to provide the output pixel format (or getting the original pixel format) and to not default to rgb. For example getting yuv420p from a video decoder will be useful in case we want to create jpegs or feed the stream to another encoder for transcoding.

This will introduce a breaking change, since by default we'll get the original pixel format. I have a working code and I can open a PR if this is something you want to include in the lib.

[mickel8]

Sure, please open the PR!