Engine: reintroduced a suspend mode in WaitForNextFrame()

Previously the engine was using allegro timers that were suspended by default when the player switched away from the game.
The commits f2ab428 and 15489ff  broke that behavior. Here we reintroduce it, now implementing the timer lock mechanic directly in the engine.

Engine/ac/game.cpp

@@ -102,6 +102,7 @@ RoomStruct thisroom;
 
 volatile int switching_away_from_game = 0;
 volatile bool switched_away = false;
+volatile bool game_update_suspend = false;
 volatile char want_exit = 0, abort_engine = 0;
 GameDataVersion loaded_game_file_version = kGameVersion_Undefined;
 int frames_per_second=40;
@@ -1399,9 +1400,9 @@ void display_switch_out()
 // Called when game looses input focus and must pause until focus is returned
 void display_switch_out_suspend()
 {
-    display_switch_out();
-
     switching_away_from_game++;
+    game_update_suspend = true;
+    display_switch_out();
 
     platform->PauseApplication();
 
@@ -1427,11 +1428,11 @@ void display_switch_out_suspend()
 // Called whenever game gets input focus
 void display_switch_in()
 {
-    switched_away = false;
     ags_clear_input_buffer();
     // If auto lock option is set, lock mouse to the game window
     if (usetup.mouse_auto_lock && scsystem.windowed)
         Mouse::TryLockToWindow();
+    switched_away = false;
 }
 
 // Called when game gets input focus and must resume after pause
@@ -1456,6 +1457,7 @@ void display_switch_in_resume()
     // TODO: find out if anything has to be done here for SDL backend
 
     platform->ResumeApplication();
+    game_update_suspend = false;
 }
 
 void replace_tokens(const char*srcmes,char*destm, int maxlen) {

Engine/ac/timer.cpp

@@ -11,13 +11,14 @@
 // http://www.opensource.org/licenses/artistic-license-2.0.php
 //
 //=============================================================================
-
 #include "ac/timer.h"
-
 #include "core/platform.h"
 #include <thread>
+#include "ac/sys_events.h"
 #include "platform/base/agsplatformdriver.h"
 
+extern volatile bool game_update_suspend;
+
 namespace {
 
 const auto MAXIMUM_FALL_BEHIND = 3;
@@ -60,6 +61,11 @@ void WaitForNextFrame()
     // early exit if we're trying to maximise framerate
     if (frameDuration <= std::chrono::milliseconds::zero()) {
         next_frame_timestamp = now;
+        // suspend while the game is being switched out
+        while (game_update_suspend > 0) {
+            sys_evt_process_pending();
+            platform->YieldCPU();
+        }
         return;
     }
 
@@ -74,6 +80,12 @@ void WaitForNextFrame()
     }
 
     next_frame_timestamp += frameDuration;
+
+    // suspend while the game is being switched out
+    while (game_update_suspend > 0) {
+        sys_evt_process_pending();
+        platform->YieldCPU();
+    }
 }
 
 void skipMissedTicks()