In preperation for having allowing drawing onto multiple onscreen
framebuffers, move the onscreen framebuffer handling to the
corresponding winsys dependent backends.

Currently the onscreen framebuffer is still accessed, but, as can seen
by the usage of "legacy" in the accessor name, it should be considered
the legacy method. Eventually only the X11 Compositing Manager backend
will make use of the legacy single onscreen framebuffer API.

https://bugzilla.gnome.org/show_bug.cgi?id=768976