Fix GH-17349: Tiled truecolor filling looses single color transparency

This is porting the relevant part of a previous upstream commit[1] to
align the behavior of our bundled libgd with

Fix GH-17349: Tiled truecolor filling looses single color transparency

This is porting the relevant part of a previous upstream commit[1] to
align the behavior of our bundled libgd with upstream. It should be
noted that this only works if the image actually has a transparent
color.

[1] <https://github.com/libgd/libgd/commit/4770e2b2d50c71e60cb073f6bc2e9bc82ec978f7>

Closes GH-17351.

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