Fix #79615: Wrong GIF header written in GD GIFEncode

The color resolution is expected in bits 4-6 of the packed fields byte
of the logical screen descriptor (byte 10 of the GIF data stre

Fix #79615: Wrong GIF header written in GD GIFEncode

The color resolution is expected in bits 4-6 of the packed fields byte
of the logical screen descriptor (byte 10 of the GIF data stream),
according to the specification[1], section 18.

[1] <https://www.w3.org/Graphics/GIF/spec-gif89a.txt>

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