Add a way to manually adjust netplay delay

[Calinou]

RMG currently uses automatic delay in netplay. This is good for ease of setup, but in games where low latency is critical (such as fighting games), this can lead to sudden changes in the delay which makes the experience inconsistent (and less fair). Therefore, setting the delay manually is preferred. When you know all players are on a wired connection, it's possible to achieve lower, more consistent delay this way.

For reference, in Dolphin, the host can define a minimum delay for all players (which is automatically enforced on other clients when changed during gameplay) and players can manually increase it for themselves (in case their connection is worse than their opponent's):

And you can see each player's configured delay:

The minimum delay option here is the most important one, as different players will generally use the same delay in fighting games to ensure fairness.

Having these options would make RMG more viable to play Smash 64 online (we currently use Project64KSE, which is very old and quirky in comparison).

[Rosalie241]

You can see the general design goals of the netplay in mupem64plus-core here: mupen64plus/mupen64plus-core#753

The game will "punish" the player with a poor internet connection (they may experience higher input latency and missed input, but it won't affect the other players). I think this is an important principle in online gaming

There was some work to implement configurable input delay here: mupen64plus/mupen64plus-core#819

But that went nowhere sadly.