Fix GH-17373: imagefttext() ignores clipping rect for palette images

We apply the same fix that has been applied to external libgd at least
as of 2.0.29.

To avoid issues regardi

Fix GH-17373: imagefttext() ignores clipping rect for palette images

We apply the same fix that has been applied to external libgd at least
as of 2.0.29.

To avoid issues regarding minor FreeType rendering differences, the
test case does not compare against an image, but rather checks that all
pixels outside the clipping rect have the background color.

Closes GH-17374.

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