In the `OnKeyDown` event handler, check if the key is actually pressed
using `wxGetKeyState()`, because for some reason on Ubuntu 18
spurious events are generated when the key is not actually pressed.
Also, if a game is paused by the frontend while a key is being pressed,
the key will remain active for the game when unpaused. This is an
issue because the key will likely be released outside of the game zone;
therefore, we would not process the key release for the game itself.
The same bug happens if clicking outside of the game zone while
holding a key.
For the case of a directional key, this means a continuous movement
in the direction of the key pressed before the pause until the key
is pressed and released.
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