multimedia/ffmpeg: Update default OPTIONS for THEORA, XVID, WEBP

Details:

• Remove THEORA and XVID from default OPTIONS: Both formats can be decoded using ffmpeg's internal decoders. For encoding, both formats are superseded for almost all practical purposes by H264,H265,AV1,etc.
• Modify _DESC for THEORA and XVID to make it more clear that this is only required if someone wants to use those libs for encoding.
• Add WEBP (ubiquitous image format) to default OPTIONS.

PR: 261752
Reported by: jbeich