Introduce DrawingContext View superclass

In order to properly implement ClipToPicture in BView, we need to render
a Picture to a Bitmap. This is currently done client-side, but the
overhead for this (creating a BBitmap that accepts views, including a
window thread, adding a view to it, and rendering the picture to the
view, then sending the result to app_server) isn't acceptable. Moreover,
the bitmap drawn this way is clipped to the view size, and the clipping
won't work when the view is scaled or translated. So, we need to move
the Bitmap creation server-side.

However, app_server currently have no means of doing this. Factor out the
relevant parts of View: a DrawingState stack with PushState/PopState, a
DrawingEngine, and a ConvertToScreen transformation. Another implementation
of the DrawingContext will allow us to also draw a picture directly using
a Painter and low-level pixel buffer, in a format suitable for use as an
AGG alpha mask.
Fix build.