Fix generation of large PDFs

[clementmas]

Browsershot can not currently save PDFs that are over 512 MB.

Browsershot::url('https://page-containing-large-images.com')->save('large-file.pdf');

The issue is described quite well here: microsoft/playwright#14675

The error Cannot create a string longer than 0x1fffffe8 characters occurs at output.toString('base64') which can not handle very large output (see Stack Overflow thread).

However, we don't need to convert the output to base64 when saving PDF files to disk. If we did, we should serialize the output to buffer but I don't think this is necessary.

Fixes #642 and #745.

Note: this is the same PR as #779 but reopened to fix the git rebase issue

[sweptsquash]

Sorry for being direct, but is there any update on this PR @freekmurze I'm also facing a similar issue.

[freekmurze]

Thanks!