[AdgLineStyle] Added guard against invalid properties where possible
Check "width" and "miter-limit" values are greather or equal than 0
before accepting them.
The enum properties, instead, are provided by cairo and there is no way
to check for invalid values other than hardcode the current values
inside the ADG project. This would disallow the use of new values
introduced by future cairo releases, hence not an acceptable solution.
Bottom line: values needed by cairo enum properties are passed as is.