Request: Pointer grabbing and capturing

[diamondburned]

Use-case: remote gaming, same as any1/neatvnc#38.

I think most remote gaming needs pointer grabbing and capturing, so I'm making
this issue as a feature request for that. Steam Remote Play currently has
support for that, but it's really buggy and doesn't support Wayland.

I'm not sure if a new extension is needed, but issue
TigerVNC/tigervnc#619 could be relevant.

[any1]

To support the method chosen by tigervnc, we would have to implement the vmware extension for cursor position feedback, but that seems rather fragile with regard to rtt. It seems that the qemu extension mentioned in the issue might be a better fit.