Removed pen adjustment for Cairo >= 1.12

Modern systems have a Cairo library where pen adjustment is no longer necessary.
This fixes errors in the Mono System.Drawing testsuite caused by wrong adjustment.

The change should have no effect on Mac OSX, as the Mono bockbuild there uses a modern version of cairo and pen adjustment was disabled already with b2fd6394f5604f2f6de66f07935b64ed575af48c.

As discussed in #monodev.