Cairo API additions in Context class

These commits add 3 methods to the Context class, coming from the cairo-sharp binding in Gtk#.

They are not strictly necessary for Gtk#, but have been in cairo-sharp for a while. So I'm proposing their addition to Mono.Cairo, to align the API of the 2 assemblies.

The end goal is to have Gtk# default back to Mono.Cairo when built with Mono.