Fix bug 68629: Transparent artifacts when using imagerotate

We port the respective upstream fix[1], which dropped the special cased
implementations of fixed-point arithmetic rotation in

Fix bug 68629: Transparent artifacts when using imagerotate

We port the respective upstream fix[1], which dropped the special cased
implementations of fixed-point arithmetic rotation in favor of the
generic implementation.

We also port follow-up upstream fixes[2][3].

[1] <https://github.com/libgd/libgd/commit/bd6d2e101f6f1df106d1cd2e2dc8058a5538109b>
[2] <https://github.com/libgd/libgd/commit/6d21d304295731d67db324dc3bf4630a69ebc5d6>
[3] <https://github.com/libgd/libgd/commit/9df878a4006130f3b5c8ee6e1f5ce9e8e6c9db07>

Closes GH-17375.

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