The AGI engine's save/load dialog dirties the entire screen thirteen times

each iteration of the loop, causing a serious slowdown on my computer. It
empathically should not be doing that, of course, but to safe-guard against
such patological cases, if the entire screen is dirtied even once we can set
_forceFull to true.

svn-id: r22616

backends/sdl/graphics.cpp

@@ -907,6 +907,11 @@ void OSystem_SDL::addDirtyRect(int x, int y, int w, int h) {
 	}
 #endif
 
+	if (w == width && h == height) {
+		_forceFull = true;
+		return;
+	}
+
 	if (w > 0 && h > 0) {
 		SDL_Rect *r = &_dirtyRectList[_numDirtyRects++];