Fix GH-10344: imagettfbbox(): Could not find/open font UNC path

libgd uses an incorrect absolute path check in gdft.c.
It checks if either the path starts with a '/' (only valid on Posix

Fix GH-10344: imagettfbbox(): Could not find/open font UNC path

libgd uses an incorrect absolute path check in gdft.c.
It checks if either the path starts with a '/' (only valid on Posix
btw), or whether it contains something of the form C:\ or C:/.
However, this overlooks the possibility of using UNC paths on Windows.
As we already do PHP-specific stuff with VCWD_ macros, use
IS_ABSOLUTE_PATH to check for an absolute path which will take into
account UNC paths as well.

Closes GH-13241.

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