PDF: Support the poppler 0.62 renderToImage with update callback

Summary:
This way pages that take more than 500ms to render get updated every so often so that the
user can see that the program didn't hang, it's just that it's taking long to render

Tags:
incremental rendering, partial updates

BUGS: 344081

Subscribers: #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D8379