We have both NativeSurfaceType::CAIRO_CONTEXT and
NativeSurfaceType::CAIRO_SURFACE. They both refer to DrawTargets with Cairo
backends. The only reason for having both is so you can call GetNativeSurface()
and get either the |cairo_t*| or the underlying |cairo_surface_t*|.
I argue that this convenience confuses things more than it helps. This patch
removes CAIRO_SURFACE, and adds explicit cairo_get_target() calls where
necessary.
