Fix continuous sprinting when using a modifier key

as a mapping key (#142).

When holding a key(W,A,S,D) and then pressing a modifier(SHIFT,CTRL,ALT),
we get two sets {MOD,MOD,0} and {KEY,MOD,0} with MOD > 0 after a
{KEY,0,0}. When releasing the initial key while holding the modifier,
we search for {KEY,MOD,0} instead of {KEY,0,0} to be removed.

Finally, when releasing the modifier, we remove {MOD,MOD,0} from our
stack. This means the {KEY,0,0} stays "pressed", although no input
is being taken from the keyboard.

Full example using WASD as movement keys and L_SHIFT as our B button.
1. Press and hold "A" --> add {A,0,0} to stack;
2. Press "L_SHIFT" --> add {L_SHIFT,4,0} and {A,4,0} to stack;
3. Release "A" --> remove {A,4,0} from stack (no effect);
4. Release "L_SHIFT" --> remove {L_SHIFT,4,0} from stack;
5. {A,0,0} still on stack.

No keyboard button is being pressed anymore, but there is a continuous
movement towards the direction mapped by "A" {A,0,0}. It is only fixed
when "A" is pressed two times in succession, because we don't add
duplicate sets to our stack, but try to remove them anyway.

src/wx/panel.cpp

@@ -1183,6 +1183,9 @@ static bool process_key_press(bool down, int key, int mod, int joy = 0)
             mod |= wxMOD_RAW_CONTROL;
             break;
 #endif
+        default:
+            mod = 0;
+            break;
     }
 
     // check if key is already pressed