adg: set direction in adg_ldim_new_full()
build: compile demo before doc

Conveniently builds demo programs before i18n and doc stuff, so I do not
need to wait until the end for running adg-demo-uninstalled.
adg: cleaned up adg_entity_point()

Refactored code to look cleaner. It *should* do the same job as before.
adg: improved AdgPoint handling

Cleaned up API, refactored code, return a newly CpmlPair from
adg_point_get_pair() and added test cases.
doc: corrected typos in git annotations
adg: avoid to screw the cairo context on invalid data

Setting a non-invertible cairo matrix on a cairo context will result in
an invalid context: all subsequent cairo API calls will fail. This is by
design, and there is no way to clear the status:

http://lists.cairographics.org/archives/cairo/2008-March/013433.html

Correction of the bug above is in the cairo TODO list.

Added some validation code to avoid most common mistakes, i.e.
coincident points.