Reducing resource requirements

Problem/Motivation

Big PDF file conversions need too many resource (memory, tmp storage space).

Steps to reproduce

Generate the first 4 page to jpg from a large (100+ page) pdf file.

Proposed resolution

Maybe better to use libvips trough https://github.com/libvips/php-vips

Remaining tasks

Make the module be compatible to use libvips.

User interface changes

Option in settings to use libvips instead of imagemagick.

Any thoughts?