Fix 30e69c51: palette was not marked dirty when creating a new

This means the code depended that the caller did this for us
before MakePalette() is executed, which is neither a
requirement nor a promise the code makes.

src/video/sdl2_v.cpp

@@ -95,6 +95,8 @@ static void MakePalette()
 		if (_sdl_palette == nullptr) usererror("SDL2: Couldn't allocate palette: %s", SDL_GetError());
 	}
 
+	_cur_palette.first_dirty = 0;
+	_cur_palette.count_dirty = 256;
 	_local_palette = _cur_palette;
 	UpdatePalette();