Support direct rendering from server to Chromecast

Let me tell you a story 😊

Mrs. Peabody wants to display pictures to her parents. She currently uses Google Photos. Opens the app, turns on the TV+Chromecast, presses the cast button, and pictures & videos show on the TV as she swipes on them in her phone. She has some 60fps videos from her last snowboarding holidays and the video renders perfectly smooth in TV (a little low-res at the beginning until buffer catches up, but nobody notices).

Mr. Peabody wants to move from Google to Nextcloud - for obvious reasons -, but cannot do that until his wife is able to show pictures to her parents in the TV comfortably. This includes:

- 1 button press to cast.
- Direct nextcloud-to-chromecast streaming, or the video will not display smoothly.
- Pics & videos should be correctly sorted.
- Swiping to next media should cast it automatically.

Right now only screen mirroring works, but it's clunky.

Can we help Mr. Peabody?

### Actual behaviour
- No Chromecast icon.
- Video is downloaded instead of played (https://github.com/nextcloud/android/issues/379).

### Expected behaviour
- Chromecast icon on screen when casting is possible.
- When casting is on, media streamed directly from nextcloud server.