adg: avoid to screw the cairo context on invalid data

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.