Embed alpha value to gif->canvas

[sam20908]

Hello, I am using this library to load GIFs to pass frames to SFML. As the title says, if the alpha value exists in gif->canvas, then I can just directly make SFML use the data within gif->canvas and avoid this intermediate step:

          auto color = gif->canvas;
          auto buf = frame_buf;
          for (int i = 0; i < gif->height; ++i) {
            for (int j = 0; j < gif->width; ++j, color += 3, buf += 4) {
              buf[0] = color[0];
              buf[1] = color[1];
              buf[2] = color[2];
              buf[3] = 255 * !gd_is_bgcolor(gif, color);
            }
          }