Screen annotations whose rendition action resolves to an embedded audio/video
file now play through the same play-button overlay as RichMedia.
Factor the shared resolution logic into a MediaAnnotation base (used by both RichMedia and Screen).
It fixes#6078 and #2787.
Render `/Subtype /RichMedia` annotations so embedded video and audio can
be played in the viewer.
The core layer parses the `RichMediaContent` dictionary to locate the
primary playable asset and its MIME type. The display layer overlays a
play button on the annotation's poster; clicking it swaps in a
`<video>`/`<audio>` element backed by a `blob:` URL. Presentation mode
lets events reach the media controls instead of advancing the page.
It fixes#2787.
