ext/gd minor corrections concerning image format id for errors.

[devnexen]

No description provided.

[Girgias]

Makes sense to me, but the PR title seems unrelated to the code changes?

[devnexen]

The tn arguments refers to the image type for the warnings like

		php_error_docref(NULL, E_WARNING, "Passed data is not in \"%s\" format", tn);

So not formely image type id but just for this purpose if that makes sense.

[Girgias]

The tn arguments refers to the image type for the warnings like

		php_error_docref(NULL, E_WARNING, "Passed data is not in \"%s\" format", tn);

So not formely image type id but just for this purpose if that makes sense.

Ah right