Set minimum window size (to 320 x 240)

This sets the minimum window size (to 320 x 240), so that the window
cannot be resized to lower than that.

This is because there's no utility in having a game window smaller than
that, and it provides a bonus convenience of being able to resize the
game to exactly 320x240 without needing to be exactly precise about it.

This idea was suggested by Dav999.

desktop_version/src/Screen.cpp

@@ -69,6 +69,8 @@ void Screen::init(const struct ScreenSettings* settings)
     SDL_SetWindowTitle(m_window, "VVVVVV");
 #endif
 
+    SDL_SetWindowMinimumSize(m_window, SCREEN_WIDTH_PIXELS, SCREEN_HEIGHT_PIXELS);
+
     LoadIcon();
 
     badSignalEffect = settings->badSignal;