Count pixels in segment without decoding video data

**Is your feature request related to a problem? Please describe.**
Current API for counting pixels in segments does actual video stream decoding. This API is used by broadcaster, and will make it to be CPU constrained instead of being memory constrained.

**Describe the solution you'd like**
Each video frame could potentially have a different resolution, so we can't just multiply video length by frame rate. But we could potentially parse bitstream, extracting needed information but not decoding video itself. Looks like FFmpeg has an internal API for this but it isn't exposed for library consumers, so FFmpeg should be forked to expose this API.

**Describe alternatives you've considered**
Write own bitstream parser, but this is not a good option.

Start by locating the current API for counting pixels in segments and reviewing how it triggers video stream decoding. Then investigate the FFmpeg internal API mentioned in the issue and determine how it could be exposed to library consumers; done means counting pixels across varying frame resolutions without decoding video data.

Written by the indexing model from the issue text.