Throttle the client to 1000 FPS.

This is more than enough for everyone and prevents wasting CPU time.
Without this change as many client frames as possible are rendered,
Quake II uses a complete core.

src/backends/unix/system.c

@@ -99,6 +99,12 @@ Sys_Milliseconds(void)
 	return curtime;
 }
 
+void
+Sys_Sleep(int msec)
+{
+	usleep((unsigned int)1000 * msec);
+}
+
 void
 Sys_Mkdir(char *path)
 {

src/backends/windows/system.c

@@ -429,6 +429,12 @@ Sys_Milliseconds(void)
 	return curtime;
 }
 
+void
+Sys_Sleep(int msec)
+{
+	Sleep(msec);
+}
+
 /* ======================================================================= */
 
 static qboolean

src/client/cl_main.c

@@ -807,7 +807,19 @@ CL_Frame(int msec)
 			miscframe = false;
 		}
 
-		// TODO: Do we need the cl_sleep stuff?
+		// Throttle the game a little bit. 1000 FPS are enough.
+		if (!packetframe && !renderframe && !cls.forcePacket && !userinfo_modified)
+		{
+			double frametime = (1000.0 / cl_maxfps->value - packetdelta) <= (1000.0 / gl_maxfps->value - renderdelta) ?
+							   (1000.0 / cl_maxfps->value - packetdelta) : (1000.0 / gl_maxfps->value - renderdelta);
+
+			if (frametime > 1)
+			{
+				Sys_Sleep(1);
+			}
+
+			return;
+		}
 	}
 	else if (msec < 1)
 	{

src/common/header/common.h

@@ -764,6 +764,7 @@ void Sys_Error(char *error, ...);
 void Sys_Quit(void);
 char *Sys_GetHomeDir(void);
 const char *Sys_GetBinaryDir(void);
+void Sys_Sleep(int msec);
 
 void Sys_FreeLibrary(void *handle);
 void *Sys_LoadLibrary(const char *path, const char *sym, void **handle);