RFR: 8356137: GifImageDecode can produce opaque image when disposal method changes

[Jeremy Wood]

This resolves a gif parsing bug where an unwanted opaque rectangle could appear under these conditions:

1. The disposal method for frames is 1 (meaning "do not dispose", aka "DISPOSAL_SAVE")
2. The transparent pixel is non-zero
3. There's more than one such consecutive frame

Previously: the GifImageDecoder would leave the saved_image pixels as zero when they were supposed to be transparent. This works great if the transparent pixel index is zero, but it flood fills the background of your frame with the zeroeth color otherwise.

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Commit messages:
 - 8356137: Fixing non-zero transparent pixel with disposal method 1
 - Merge branch 'master' of https://github.com/mickleness/jdk
 - Merge branch 'master' of https://github.com/mickleness/jdk
 - Merge branch 'master' of https://github.com/mickleness/jdk
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge pull request #5 from openjdk/master
 - Merge pull request #4 from openjdk/master
 - Merge pull request #3 from openjdk/master
 - ... and 2 more: https://git.openjdk.org/jdk/compare/898d4798...5dbf7a11

Changes: https://git.openjdk.org/jdk/pull/25044/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25044&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356137
  Stats: 155 lines in 3 files changed: 155 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25044.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25044/head:pull/25044

PR: https://git.openjdk.org/jdk/pull/25044