make frame skip a preference

good values are
 1 - render only even frames
 3 - render only frames divisble by 4 (i.e., only 25%)

anything higher is probably just choppy

src/include/unit.h

@@ -469,6 +469,8 @@ public:
 	bool HardwareCursor;    /// If true, uses the hardware to draw the cursor. Shaders do no longer apply to the cursor, but this way it's decoupled from the game refresh rate
 	bool SelectionRectangleIndicatesDamage; /// If true, the selection rectangle interpolates color to indicate damage
 
+	int FrameSkip;          /// Mask used to skip rendering frames (useful for slow renderers that keep up with the game logic, but not the rendering to screen like e.g. original Raspberry Pi)
+
 	int ShowOrders;			/// How many second show orders of unit on map.
 	int ShowNameDelay;		/// How many cycles need to wait until unit's name popup will appear.
 	int ShowNameTime;		/// How many cycles need to show unit's name popup.

src/stratagus/stratagus.cpp

@@ -405,7 +405,7 @@ void Exit(int err)
 	FreeButtonStyles();
 	FreeAllContainers();
 	freeGuichan();
-	fprintf(stdout, "Frames %lu, Slow frames %d = %ld%%\n",
+	fprintf(stdout, "Frames %lu, Slow frames %ld = %ld%%\n",
 			   FrameCounter, SlowFrameCounter,
 			   (SlowFrameCounter * 100) / (FrameCounter ? FrameCounter : 1));
 	lua_settop(Lua, 0);

src/tolua/unit.pkg

@@ -37,6 +37,8 @@ class CPreference
 	bool HardwareCursor;
 	bool SelectionRectangleIndicatesDamage;
 
+        unsigned int FrameSkip;
+
 	unsigned int ShowOrders;
 	unsigned int ShowNameDelay;
 	unsigned int ShowNameTime;

src/video/sdl.cpp

@@ -719,9 +719,6 @@ const EventCallback *GetCallbacks()
 	return Callbacks;
 }
 
-static int SkipFrameMask = 0;
-static unsigned long NextSlowFrameReaction = FRAMES_PER_SECOND * 10;
-
 /**
 **  Wait for interactive input event for one frame.
 **
@@ -741,23 +738,6 @@ void WaitEventsOneFrame()
 	Uint32 ticks = SDL_GetTicks();
 	if (ticks > NextFrameTicks) { // We are too slow :(
 		++SlowFrameCounter;
-		if (SlowFrameCounter > NextSlowFrameReaction) {
-			unsigned long pct = (SlowFrameCounter * 100) / (FrameCounter ? FrameCounter : 1);
-			if (pct >= 60) {
-				SkipFrameMask = 0b111;
-			} else if (pct >= 40) {
-				SkipFrameMask = 0b101;
-			} else if (pct >= 20) {
-				SkipFrameMask = 0b11;
-			} else if (pct >= 10) {
-				SkipFrameMask = 0b1;
-			}
-			NextSlowFrameReaction += FRAMES_PER_SECOND * 10;
-			fprintf(stdout, "WARNING WARNING WARNING\n"
-							"Frames %lu, Slow frames %d = %lu%%, starting to render only every %d%s frame.\n",
-							FrameCounter, SlowFrameCounter, pct, SkipFrameMask + 1, SkipFrameMask == 1 ? "nd" : "th");
-			fflush(stdout);
-		}
 	}
 
 	InputMouseTimeout(*GetCallbacks(), ticks);
@@ -855,7 +835,7 @@ void RealizeVideoMemory()
 	if (dummyRenderer) {
 		return;
 	}
-	if (FrameCounter & SkipFrameMask) {
+	if (Preference.FrameSkip && (FrameCounter & Preference.FrameSkip)) {
 		return;
 	}
 	if (NumRects) {