Defining the default draw target avoids the previous mix of cairo
and d2d layers, which would trigger an assertion when capturing
a page with the widget layers.
Supports creating a windowless browser on Linux without an X server. Most of the
changes are just adding branches to avoid calls in to GTK which calls
into X. Some of the bigger additions were adding a separate headless widget
which implements just enough to render a page. A headless look and
feel were also added since there are many calls into GTK in the platform
specific one.
