Issue with ImagePNG XS

[nickgl]

Hello,

This issue relates to the optional ImagePNG XS module.

In PDF::API2::Resource::XObject::Image::PNG there's a line which attempts to determine if the XS module is available, using eval.

https://github.com/ssimms/pdfapi2/blob/master/lib/PDF/API2/Resource/XObject/Image/PNG.pm#L18-L19

However, the current imagePNG.pm doesn't appear to parse:

https://metacpan.org/release/SSIMMS/PDF-API2-XS-1.002/source/lib/PDF/API2/XS/ImagePNG.pm#L11-12

$ perl -c PDF/API2/XS/ImagePNG.pm
Global symbol "@ISA" requires explicit package name (did you forget to declare "my @ISA"?) at PDF/API2/XS/ImagePNG.pm line 11.
Global symbol "@EXPORT" requires explicit package name (did you forget to declare "my @EXPORT"?) at PDF/API2/XS/ImagePNG.pm line 12.
PDF/API2/XS/ImagePNG.pm had compilation errors.

I believe we want 'our' adding to these two lines. This is masked by the eval, so we're not seeing the benefts of the XS module.

Although in itself this isn't an issue, I'm working on a project where there's an sigdie handler, and that's how we spotted it.