Manage resetting the key event state on video reinitialization centrally.

Manage resetting the key event state on video reinitialization centrally.

Fixes:
- No key state clear when SDL gets restarted.
- Key state clear was the wrong action - it allows all keys to get pressed
  again, but does not actually release anything in cl_input.c, so e.g. +jump
  binds stay pressed on vid_restart.

Only vid_wgl.c retains some vid-specific handling (reset of key state when
alt-tabbing out and back in) - there's a chance that we may need similar logic
in the other vid drivers; if so, it may be possible to do this in a shared
place too by watching for changes to vid_activewindow.