Fix boxgallery for simplex documents.
Check that the surrounding of a line is not dark.

Before there was no check that a line really was that.
The follow_line function could also find eg. the edge
of a large black area.
Add API to find corner markers directly.

This may be useful in some scripts, even though SDAPS
does not currently use it itself.
Add API to save an A1 surface to a TIFF file.

The API adds a new page to an existing TIFF file, or
creates a new file. It can be used to create a TIFF
that can later be opened by SDAPS.
Ensure found corners are not on the edge of the image.
Do not try to reload dummy images.
Check that the marker is a corner.

This works by checking that two endpoints are close to each other.
Calculate a quality for the checkbox position match.
Fix cline command inside choicequestion.

Apparently having the macro to generate the dummy row after
the start of the tabularx creates some issues. This moves
the macro in front of the tabularx. With this \cline works
in front of the first (user created) line.