Implement relative mouse mode emulation using cursor centering

[rdb]

Description

Implement relative mouse mode emulation by using confined mode with automatic cursor centering, if other methods fail.

Use Case

At the moment, if neither libxi nor xxf86dga are installed on Linux, relative mouse does not work. This can be worked around but this is a rare occurrence so developers are not prepared for it.

If we implemented a fallback, developers can always count on the mode working and do not have to implement fallback cases themselves.

[rdb]

Apparently, cursor warping is not supported on XWayland, making it not a reliable fallback option. So this fallback would only work on proper X.Org. I don't have an X.Org installation so it's hard for me to implement this.

I'm wondering if instead we should be statically linking libXi, or manually implementing those functions somehow.