Faster image and deeplink export

[marcharding]

This improves speed when images and deeplinks are exported.

In my test cases the export now takes about 50% less time and uses 10% less memory than before (ymmv).

[tuegeb]

@xpoback: Could you check whether this improvement could be integrated into the version 4.x, please?

[xpoback]

@marcharding Many thanks for your contribution!
@tuegeb I integrated this to the current version, also added saving the image, since it was missing.
At my test instance it used about 50% less time and about 15% less memory (PHP7.0), especially in cases when no cache images were created yet.

[xpoback]

This should also fix the issue in #173

[tuegeb]

@xpoback: I can confirm that it solves #173, too. Many thanks.