Center cursor on startup

[pwnfooj716]

This is a super minor annoyance, but it would be nice if the cursor would start in the center of the screen when you first launch river. This could be the default behavior or it could be implemented as a riverctl command that I can run in my startup script. Currently, there is no riverctl command to directly move the mouse cursor except for the "set-cursor-warp" settings which don't apply in this case. Even setting cursor-warp to "on-focus-change" doesn't work to center the cursor when launching my first application (is that a bug?).

I'd be willing to help with implementing this change if someone could direct me to the right places in the code to look.