Fix 52317bb7: [SDL2] ensure we don't try to blit out of bounds (#8684)

During resizing, there can still be dirty-rects ready to blit based
on the old dimensions. X11 with shared memory enabled crashes if
you try to do this. So, instead, if we resize, reset the dirty-rects.

This is fine, as moments later we mark the whole (new) screen as
dirty anyway.

src/video/sdl2_v.cpp

@@ -351,6 +351,13 @@ bool VideoDriver_SDL::CreateMainSurface(uint w, uint h, bool resize)
 		_sdl_surface = _sdl_real_surface;
 	}
 
+	/* X11 doesn't appreciate it if we invalidate areas outside the window
+	 * if shared memory is enabled (read: it crashes). So, as we might have
+	 * gotten smaller, reset our dirty rects. GameSizeChanged() a bit lower
+	 * will mark the whole screen dirty again anyway, but this time with the
+	 * new dimensions. */
+	_num_dirty_rects = 0;
+
 	_screen.width = _sdl_surface->w;
 	_screen.height = _sdl_surface->h;
 	_screen.pitch = _sdl_surface->pitch / (bpp / 8);