direct mode ought probably have a streaming capability

[dankamongmen]

[shrug] while i don't think it a particularly great idea, there's no real reason why direct mode ought not have a stream() capability, i guess (this arises due to work on #1342). i would think that direct mode streaming would:

• save, and explicitly return to, the same position before drawing each frame
• be entirely unoptimized at the rasterization level unless we wanted to manually detect damage, ugh
• need to somehow cope with scrolling, should scrolling occur

like i said, i don't think this is the best idea in the world. the lack of optimization in particular might cast us in a bad light. if someone comes up with a valid use for this, though, it'll be good to have thought about it.

[dankamongmen]

this will, at minimum, require changing up the media API, since e.g. ffmpeg_stream() requires a notcurses* at the moment.