Fix: Make cursor position more consistant

[Astatin3]

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What this does:

• Cursor is now initialized in the center of the output when a seat is created (A small pet peeve of mine with Cosmic)
• When a output is removed, and the cursor is inside of it, it is set to the center of the 0th index output (see the comment at src/shell/mod.rs:940)
• Keeps track of the relative position of the cursor to the output when monitors are connected or removed, the cursor isn't teleported about the screen because the geometry changed

I was only able to test this on Wayland, with one external monitor, some more testing would definitely be appreciated!