Add a very crude screenshot function (press F3)

Files are named after the current date+time (down to seconds),
and written to the gameFolder directory.

src/eventthread.cpp

@@ -185,6 +185,9 @@ void EventThread::process(RGSSThreadData &rtData)
 				break;
 			}
 
+			if (event.key.keysym.scancode == SDL_SCANCODE_F3)
+				rtData.rqScreenshot = true;
+
 			keyStates[event.key.keysym.scancode] = true;
 			break;