Enable option to set thread count on FFmpegVideoEncoder.

[Todilo]

• Enable option to set thread count on FFmpegVideoEncoder.
• Make SetBitrate public, internal won't let users of other sources than FFmpegVideoSource to utilize it.

[ha-ves]

It might be too low-level and confusing to the user to operate directly on the encoder-level because SIPSorceryMedia.Abstractions?

Can you provide interface method for operations (along with checks and exception guarding) in FFmpegVideoSource?

[ChristopheI]

Thank you for this PR !