previously poppler_page_get_annot_mapping() function
was ignoring to rotate annots flagged as 'flagNoRotate'
probably because they require a special rotation where
the annotation pivots on the upper left corner, as
described in PDF 1.7 spec "8.4.2 Annotation Flags"
NoRotate flag.

Added support for that, while refactoring the code to
make it more readable.

Fixes #767