input: Propagate modifiers to clients with only pointer focus

[danieldg]

When focus_follows_mouse is off, it is possible to hover and/or scroll windows without giving them keyboard focus. However, attempts to use Ctrl+scroll on such clients will not work correctly because the keyboard modifiers are not available. This patch propagates modifier state to the client with pointer focus ~~when it gains focus and when the state changes~~ before sending axis events.

https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/259 discusses permitting this behavior at the protocol level.