SDLBackend: do not leak Super+KEY shortcuts to DE

[pchome]

Some gamescope shortcuts are DE default shortcuts.

For example Super+I is KDE Plasma Settings shortcut, and Super+B is Power Profile Switch in Plasma Power Management.

Funny enough Super+G is the default shortcut too, but keyboard grab seems resets the key modifier (?) so it not detected by DE. Also user must press the Super key again after that for other gamescope shorts to work.

Maybe a bit hackky solution is to temporary grab keyboard (if not already grabbed) while the Super key remain pressed and release grab when the key released.

During the key grab we lose key modifier (as was mentioned above), so the same toggle-variable used as alternative modifier indicator.

NOTE: DE will not receive Super+KEY shortcuts in this case. If there were some important ones for users (e.g. Super+PrintScreen).

Closes: #2156

[pchome]

To debug SDL events use SDL_EVENT_LOGGING=1 (or =2 for more events).
For example SDL_EVENT_LOGGING=1 gamescope -F fsr -- vkgears

...
SDL EVENT: SDL_EVENT_KEY_DOWN (timestamp=5926131533 windowid=2 which=0 state=pressed repeat=false scancode=227 keycode=1073742051 mod=0x400)
SDL EVENT: SDL_EVENT_KEY_UP (timestamp=6190009554 windowid=2 which=0 state=released repeat=false scancode=227 keycode=1073742051 mod=0x0)
...

[awsms]

Would be better to have user-defined keybinds than stealing a key globally.
This would just mean breaking any system-defined keybinds because gamescope cares so much about hardcoding keybinds..

[pchome]

Yeah, I guess.
Using this patch for a while and there are some cases where it not work as expected.
For example Super (Meta in KDE interpretation) is the shortcut itself (for Application Launcher). If something stealing focus while Super key is pressed then on key release system receives KEY_UP and opens menu.

So this solution is rather bad, PR can be closed if no other plans.