Add mutex to prevent borrowing error

[1995parham]

Resolve #10

I have added mutex to prevent multiple mutable reference at the same time, and it works.

[danyspin97]

The idea of using a mutex is corret, we just need to wrap surfaces with a mutex instead of RefCell in https://github.com/danyspin97/wpaperd/blob/main/src/main.rs#L84.

[1995parham]

Sure, I will change it to use mutex on surfaces.

[1995parham]

I tried to use Mutex instead of RefCell, I hope it is okay :see_no_evil:

[danyspin97]

Yea, it is okay. Thank you for your PR! 😄