Keep rendering while unfocused (#1638)

In windowed mode, we shouldn't stop rendering just because we have lost
focus. However, we don't need to render at full framerate; throttling to
5 fps should be sufficient. Note though that we still have to calculate
game ticks at full speed so network games don't slow down when a player
tabs out of the game.

src/game/C4GraphicsSystem.cpp

@@ -34,6 +34,8 @@
 
 #include <StdPNG.h>
 
+static const int MAX_BACKGROUND_FPS = 5;
+
 C4GraphicsSystem::C4GraphicsSystem()
 {
 	Default();
@@ -68,9 +70,10 @@ bool C4GraphicsSystem::StartDrawing()
 	if (!pDraw) return false;
 	if (!pDraw->Active) return false;
 
-	// only if application is active or windowed (if config allows)
-	if (!Application.Active && (!Application.isEditor || !Config.Graphics.RenderInactiveEM)) return false;
-
+	// if the window is not focused, draw no more than MAX_BACKGROUND_FPS frames per second
+	if (!Application.Active && (C4TimeMilliseconds::Now() - lastFrame) < 1000 / MAX_BACKGROUND_FPS)
+		return false;
+	
 	// drawing OK
 	return true;
 }
@@ -78,6 +81,7 @@ bool C4GraphicsSystem::StartDrawing()
 void C4GraphicsSystem::FinishDrawing()
 {
 	if (!Application.isEditor) FullScreen.pSurface->PageFlip();
+	lastFrame = C4TimeMilliseconds::Now();
 }
 
 void C4GraphicsSystem::Execute()

src/game/C4GraphicsSystem.h

@@ -60,7 +60,8 @@ public:
 	bool DoSaveScreenshot(bool fSaveAll, const char *szFilename, float fSaveAllZoom);
 	inline void InvalidateBg() { iRedrawBackground=2; }
 	inline void OverwriteBg() { InvalidateBg(); }
-protected:
+
+private:
 	char FlashMessageText[C4MaxTitle+1];
 	int32_t FlashMessageTime,FlashMessageX,FlashMessageY;
 	void DrawHelp();
@@ -68,6 +69,8 @@ protected:
 	void DrawHoldMessages();
 	void ClearFullscreenBackground();
 
+	C4TimeMilliseconds lastFrame;
+
 public:
 	bool ToggleShow8BitSurface();
 	bool ToggleShowNetStatus();