Key-up events can include the wrong keycode

[stevesims]

The current keyboard handling code keeps track of the "keycode" (ASCII) value for the last key-down event, and will use that as the keycode for the keyboard packet sent to MOS for key-up events, rather than sending the correct keycode for the key that has been release.

This is problematic, as it means that if multiple keys have been pressed down the key-up packets can indicate the wrong key has been released.

This doesn't really affect MOS's keyboard handling as that uses the VK data.

Ideally the VDP should return a keycode that equates to the key being released.

It is suspected that the current behaviour seems to be in place to ensure that an ASCII value is present when a modifier key is released