In pass-through mode, the group layer-stack's input is connected to
the backdrop.  However, when rendering the group's projection, we
want to render the stack independently of the backdrop.
Unfortunately, we can't use the stack's graph as a subgraph of two
different graphs.

To work around that, the next few commits add a mechanism for a
projectable to be notified before and after its graph is being
rendered.  We use this mechanism to disconnect the stack's graph
from the backdrop before rendering the projection, and reconnect
it afterwards.  Yep, it's ugly, but it's better than having to
maintain n copies of (each node of) the graph (each nesting level
requires an extra copy.)

This commit adds {begin,end}_render() functions to GimpProjectable.
These functions should be called right before/after rendering the
projectable's graph.