Fix GIF background color index bounds

[treeform]

Fixes #560.

Summary:

• Reject GIF background color indexes that are equal to the global color table size before indexing the table.
• Add a regression test using a crafted GIF with a two-entry global color table and bgColorIndex == 2.

Tests:

• nim r --hints:off --warnings:off tests\test_gif.nim
• nim r --hints:off --warnings:off tests\tests.nim
• git diff --check HEAD~1..HEAD