Fix libgd 223: gdImageRotateGeneric() does not properly interpolate

We port the respective upstream fix[1]. We only run the test against
bundled libgd, since external libgd may yield di

Fix libgd 223: gdImageRotateGeneric() does not properly interpolate

We port the respective upstream fix[1]. We only run the test against
bundled libgd, since external libgd may yield different results.

Cf. <https://github.com/libgd/libgd/commit/2b26be874d0c0987c4906a40ef8f0e8fee04ae8f>.

Closes GH-17380.

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