Migrate skip checks to --EXTENSIONS-- for ext/gd

Cf. PR #6787.

Closes GH-6994.

Fix #79676: imagescale adds black border with IMG_BICUBIC

We have to loop over all image pixels to avoid the black border. This
is also done in external libgd in `_gdScaleOneAxis` and `

Fix #79676: imagescale adds black border with IMG_BICUBIC

We have to loop over all image pixels to avoid the black border. This
is also done in external libgd in `_gdScaleOneAxis` and `_gdScalePass`.

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