* EMF: support negative bounding box coordinates
Similar to placeable WMF, bounding box coordinates
should be interpreted as signed integer, otherwise
opening EMF file with negative (x0,y0) fails.
* Basic load tests for WMF and EMF formats
* WMF/WMF tests: just test open(), not load()
Not sure why load() fails on Debian build. Well, at least we can test
open().
* WMF/EMF: Unpack signed integers using unpack()
* WMF/EMF: Compare to reference PNG rendering
* EMF/WMF comparison: use assert_image_similar()
* Use similarity epsilon 0.5 for WMF, as vector rendering looks different across Windows platforms
* Trigger rebuild
