Add test case for GH-13774

Besides demonstrating the new behavior, this test also ensures that the
bundled and external libgd now behave the same. It has to be noted,
though, that w

Add test case for GH-13774

Besides demonstrating the new behavior, this test also ensures that the
bundled and external libgd now behave the same. It has to be noted,
though, that we only test one of the five code paths.

Closes GH-14945.

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