The Netscape looping application extension encodes how many
times the animation should restart, and if it's present
there is no way to signal that a GIF should play only once.

Use LoopCount=-1 to signal when a decoded GIF had no looping
extension, and update the encoder to omit that extension
block when LoopCount=-1.

Fixes #15768

GitHub-Last-Rev: 249744f0e28ef8907aa876070a102cb5493f5084
GitHub-Pull-Request: golang/go#23761
Change-Id: Ic915268505bf12bdad690b59148983a7d78d693b
Reviewed-on: https://go-review.googlesource.com/93076Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>