Ignore modifiers when releasing keys

If a key is pressed with a modifier and then released without that modifier (ie. ctrl+mouse2 is pressed, ctrl is released, then mouse2 is released) mpv doesn't process the key release event correctly and it thinks that the original key (with modifier) is still pressed:
```
[input] key code=0x34a000a2 'Ctrl+MOUSE_BTN2' down
[input] key 'Ctrl+MOUSE_BTN2' -> 'seek 5' in 'default'
[input] key code=0x382000a2 'MOUSE_BTN2' up
[input] key code=0x61 'a'
[input] key 'Ctrl+MOUSE_BTN2-a' -> 'cycle angle' in 'default'
```

This is reproducible on Windows. I tested with Linux/X11, but it seems to send `MP_INPUT_RELEASE_ALL` on every key up event, so the issue didn't really affect anything.