Prototype for displaying live cursor positions in matchmaking pick screen

[minetoblend]

Depends on ppy/osu-server-spectator#356

Relevant discussion on discord

RFC. Prototype for displaying user cursors on the beatmap select screen. Cursors will fade out as they approach the border of the scrollarea so they are only visible within that space.

cursors.mp4

• Currently sends a message to udpate the cursor position every 50ms, though adding batching to this should be pretty straightforward.
• One problem that I noticed when testing this is that on wider screens there's 1 extra column which would make the cursor not hover the same beatmaps as on narrower screens with only two columns. This could potentially be a deal breaker.