8380078: Update GIFlib to 6.1.2

[gnu-andrew]

Part of getting giflib updated to 6.1.2 in 8u492

This updates the in-tree giflib to 6.1.2. The actual code changes are a clean backport. The license file changes needed adapting from the giflib.md file in 11u and later to the THIRD_PARTY_README file in 8u (now only one copy following JDK-8338144)

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• JDK-8380078 needs maintainer approval
• Commit message must refer to an issue

Integration blocker

 ⚠️ Dependency #88 must be integrated first

Issue

• JDK-8380078: Update GIFlib to 6.1.2 (Bug - P3)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk8u.git pull/89/head:pull/89
$ git checkout pull/89

Update a local copy of the PR:
$ git checkout pull/89
$ git pull https://git.openjdk.org/jdk8u.git pull/89/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 89

View PR using the GUI difftool:
$ git pr show -t 89

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk8u/pull/89.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back andrew! A progress list of the required criteria for merging this PR into pr/88 will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[mlbridge]

Webrevs

• 00: Full (de466485)

[gnu-andrew]

8u492 is now frozen so will move this to 8u-dev.