Quit if Android back-button is pressed 3 times within 1 second

Interpret fast repeated presses of the back-button (3 times within 1 second) as a quit-event, so the app can be quit cleanly and quickly without using the UI. The client settings are otherwise not saved if the app is closed by minimizing it using the home button and waiting for the OS to kill it or by discarding it in the recent apps view.

src/engine/client/input.cpp

@@ -641,9 +641,35 @@ int CInput::Update()
 
 		// handle keys
 		case SDL_KEYDOWN:
+#if defined(CONF_PLATFORM_ANDROID)
+			if(Event.key.keysym.scancode == KEY_AC_BACK && m_BackButtonReleased)
+			{
+				if(m_LastBackPress == -1 || (Now - m_LastBackPress) / (float)time_freq() > 1.0f)
+				{
+					m_NumBackPresses = 1;
+					m_LastBackPress = Now;
+				}
+				else
+				{
+					m_NumBackPresses++;
+					if(m_NumBackPresses >= 3)
+					{
+						// Quit if the Android back-button was pressed 3 times within 1 second
+						return 1;
+					}
+				}
+				m_BackButtonReleased = false;
+			}
+#endif
 			Scancode = TranslateScancode(Event.key);
 			break;
 		case SDL_KEYUP:
+#if defined(CONF_PLATFORM_ANDROID)
+			if(Event.key.keysym.scancode == KEY_AC_BACK && !m_BackButtonReleased)
+			{
+				m_BackButtonReleased = true;
+			}
+#endif
 			Action = IInput::FLAG_RELEASE;
 			Scancode = TranslateScancode(Event.key);
 			break;

src/engine/client/input.h

@@ -78,8 +78,11 @@ private:
 
 	bool m_MouseFocus;
 	bool m_MouseDoubleClick;
-#if defined(CONF_PLATFORM_ANDROID) // No relative mouse on Android
+#if defined(CONF_PLATFORM_ANDROID)
-	ivec2 m_LastMousePos = ivec2(0, 0);
+	ivec2 m_LastMousePos = ivec2(0, 0); // No relative mouse on Android
+	int m_NumBackPresses = 0;
+	bool m_BackButtonReleased = true;
+	int64_t m_LastBackPress = -1;
 #endif
 
 	// IME support