Add optimized gif support for decode_gif (#16804)

* Add optimized gif support for decode_gif

While revisiting the issue of 15838, I noticed that
currently optimized gif is not supported. However,
optimized gif is actually possible to be processed
as essentially the subsequent frame just adds
the content on top of the previous frame on canvas.

This fix adds the support for optimized gif with decode_gif.

As is shown in the added test case, optimized gif (`optimized.gif`)
could be handled the same way as original gif (`scan.gif`).

This fix fixes 15838.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Format gif_io.cc with clang-format -i --style=Google

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test case to cover optimized gif support for decode_gif.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add `#include <algorithm>` to fix Windows build errors

This commit add `#include <algorithm>` to fix
build errors on Windows platform.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>