best-effort reconciliation of modifier key states from winit

[robtfm]

Objective

in some circumstances modifier key events can be swallowed by the OS and lost to the application (e.g. macos screencap key chords like super+shift+5, potentially windows gamebar chord win+g). winit updates modifier state on any relevant event and provides an event stream, but bevy does not currently use those events.

Solution

add a reconciliation vs winit's modifier events

all platforms:

• when winit declares a logical modifier is unpressed (e.g. Alt), force all the relevant keycodes (e.g. AltLeft) to unpressed. prevents "sticky modifiers" that are not caught by the blanket focus-lost clear.

macos native only (relying on keycode press state, which is only reported accurately on macos currently, but could be supported on windows if winit extends support):

• when winit declares a physical modifier is unpressed[1], force that keycode to unpressed. catches the rare case where both shifts are held and one release is swallowed.
• when winit declares a physical modifier is pressed, set the relevant key as pressed. effectively sets modifiers that are held when focus is gained.

[1]winit reports only "pressed" or "unknown". to tell if a key is declared unpressed, we check if it is "unknown" and another key in the group is "pressed"

Testing

tested with https://github.com/robtfm/modifier-test-app (on macos native and wasm).