support for single-layer Dolby Vision in HEVC/h.265 elementary streams

Earlier I intended to have all Dolby Vision related things handled via issue #2784 (closed). However, that one will be implemented soon, and supporting DV in HEVC elementary streams is a lot more work, and that work doesn't overlap with the work on the MP4 reader.

Fortunately there's MR !2232 (merged) already, laying the groundwork & going quite a way.

Note that #3093 (closed) must be solved first.