Fill the root surface with the appropriate background color - don't hardcode it in AXRRender

[jakepetroules]

AXRRender currently hardcodes the root surface fill color. It was initially transparent, and changed to white in a9a9bb0 - however, neither of these are correct.

The renderer should obtain the background color from the root object itself, and fill with whatever happens to be the default (either the default white or whatever the user may have changed it to).

[jakepetroules]

This needs a little bit more discussion about how the compositor works exactly and whose responsibilities are what.