Add something to make it easier to converting cairo contexts

[hamishmack]

Since Cairo does not have full GI we need convert gi-cairo contexts for use with the Gtk2Hs cairo package. Winds up being something like this:

import Graphics.Rendering.Cairo.Types (Cairo(..))
import Graphics.Rendering.Cairo.Internal (Render(..))

...

    withManagedPtr context $ \cPtr -> (`runReaderT` Cairo (castPtr cPtr)) . runRender $ do

Where context is the gi-cairo context. We should wrap this up into a withCairo function or something.

[garetxe]

Sounds good to me!

[jplatte]

Hm, wouldn't it make sense to create a gi-cairo-hs package that exposes the same or a similar API as the current cairo package, but is based on gi-cairo instead of Graphics.Rendering.Cairo.Internal, so we can get rid of those manually maintained low-level bindings in the long run?

[hamishmack]

@jplatte unfortunately cairo.gir is only 60 lines long (so gi-cairo contains very little), but even so gi-cairo-hs makes sense. Hopefully we could avoid depending on gtk2hs-buildtools. The same goes for gi-javascriptcore-hs.

[jplatte]

Oh, okay. What's the problem with introspecting cairo? Are the introspection comments just missing or is there another reason the gi scanner doesn't pick up most of the API?