Feat: Simplified version of my previous changes.

[davidawesome02-backup]

Implement the current changes needed for partydeck / split screen programs rebased on the latest upstream from the original branch (prerelease_david_changes).

Main changes are implementing ctrl+G keybind to grab / ungrab and add the new command line options, best described by the help page:

--libinput-hold-dev            Comma separated list of evdev path devices that will become the handlers on the nested window (WAYLAND & SDL & VR ONLY)
--backend-disable-keyboard     Disables the normal backend keyboard support (window will not handle keyboard) (WAYLAND & SDL ONLY)
--backend-disable-mouse        Disables the normal backend mouse support (window will not handle mouse) (WAYLAND & SDL ONLY)
--nested-follow-window-scale   Enables nested mode size (-w and -h) to be updated to this scale relative to the output window size (times this scale factor), when being resized. Default -1 (disabled) (WAYLAND & SDL ONLY)

Based on my other branch in the pr ValveSoftware#2106 that fixes a regression that occurred while I was not updating.