add support for Dolby Vision

Marked as WIP because it depends on the corresponding FFmpeg patch series, which is unmerged. Posting for code review.

Currently only supports the libplacebo-based video output module, though the relevant function call is abstracted so we could easily extend this to the pl_scale opengl filter as well. (Open questions for that: how to auto-add the pl_scale filter if we detect frames with dolbyvision metadata? The filter chain is already fixed before we get any frames, but we need to receive frames to detect the presence of this metadata)