Adjust the MultiplayerReturnToSize variable when manually scaling

[thearst3rd]

In multiplayer matches, using the hotkeys to change the player scale feels jittery and unreliable, and in some situations the game will prevent the player's scale from changing beyond a certain point (especially after getting grabbed or walking through a portal). I believe the reason for this is the MultiplayerReturnToSize variable which the game will use to decide what scale to gradually scale the player back to.

This pull request updates that variable whenever we manually set the player's scale. This means that using any of the three hotkeys in a multiplayer match should reliably set the player's scale to the desired scale, and the player won't gradually be returned to any other scale.

[Micrologist]

Sorry for not getting to this in a reasonable amount of time. For me to have this compile I'd have to switch out the references to Assembly-CSharp and/or Assembly-CSharp-firstpass to the post mp-update versions. I'm not sure if that breaks anything in terms of backwards compatibility, since my version of the project still references old versions of that.
Did you do any testing of that?
In the long run I'll probably end up making different releases for pre- and post-mp update, but I don't really have the time to do that right now.

[thearst3rd]

I have only tested this with the post-mp version, and unfortunately I think it's safe to say that this won't compile with any pre-mp version since this property likely doesn't exist. I don't know how preprocessing works for C# but it might be possible to check at compile time if it exists and conditionally compile that line in, but that requires two different dlls to be distributed