Description:
------------
The code to write GD2 images checks for unrecognized fmts and
adjusts them to reasonable values. However, for truecolor images
the then correct value would be further increased by 2, resulting
in an incorrect format being written to the file header.

This has been reported against libgd as
<https://github.com/libgd/libgd/issues/289>, but affects PHP's
bundled libgd as well.