Add a frame at the end showing the result for the GIF generation

[l3d00m]

The GIF endpoint could also contain a field for the game result. In PGN it's usually included in the Header or at the very end, like "{ Black wins on time. } 0-1"

This could then be shown on a final frame, as the viewer currently can only guess the result, especially if won on time. This would also remove the need to show the last frame longer manually.

In case the field in the endpoint is not set, the GIF can just be generated like it currently is.

[niklasf]

Apparently Twitter shortens the last frame, which makes it hard to see the final position. Adding another frame would work around that, too.

[niklasf]

Relevant: 8461b96.