[Cellar] define FFV1 in Matroska

[Dave Rice <[e-mail]>]

Hi cellar,

I submitted this pull request to define an FFV1 Codec Mapping for Matroska, https://github.com/Matroska-Org/matroska-specification/pull/94. Currently all FFV1 in Matroska uses the V_MS/VFW/FOURCC Mapping. In addition to what is listed below I think the V_MS/VFW/FOURCC definition may need to be extended to say that when V_MS/VFW/FOURCC is used for FFV1, and the FFV1 is version 3 or greater, then the BITMAPINFOHEADER MUST have the Configuration Record as Private Data.

Comments? Is it advantageous to define this when V_MS/VFW/FOURCC is already the practice?

Here is the text of the proposed Codec Mapping:

### V_FFV1

Codec ID: V_FFV1

Codec Name: FF Video Codec 1

Description: FFV1 is a lossless intra-frame video encoding format designed to efficiently compress video data in a variety of pixel formats. Compared to uncompressed video, FFV1 offers storage compression, frame fixity, and self-description, which makes FFV1 useful as a preservation or intermediate video format. [Draft FFV1 Specification](https://datatracker.ietf.org/doc/draft-niedermayer-cellar-ffv1/)

Initialisation: For FFV1 versions 2 or less, `Private Data` SHOULD NOT be written. For FFV1 version 3 and greater, the `Private Data` MUST contain the FFV1 Configuration Record structure, as defined in https://tools.ietf.org/html/draft-niedermayer-cellar-ffv1-01#section-4.1, and no other data.

Dave Rice