GIF encoder AVOptions:
-gifflags <flags> E..V...... set GIF flags (default offsetting+transdiff) 设置GIF标志(默认偏移+transdiff)
offsetting E..V...... enable picture offsetting 启用图片偏移
transdiff E..V...... enable transparency detection between frames 启用帧之间的透明度检测
-gifimage <boolean> E..V...... enable encoding only images per frame (default false) 启用每帧仅编码图像(默认为false)