Fix #35493: Blender crash when rendering in terminal mode

Use the same window manager for freestyle bmain as
real bmain uses. This is needed because freestyle's
bmain could be used to tag scenes for update, which
implies call of ED_render_scene_update in some cases
and that function requires proper windoew manager
to present.

Alternative would be to make render scene update does
nothing if there's no window manager, but we wanted
freestyle to be applied in rendered viewport once and
think current change is what we'll need to support
freestyle in viewport.

But a bit further, perhaps that'd make sense to not
register ED_render_scene_update as a scene update
callback when in background mode?

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56994 954f8c5b-7b00-dc11-b283-0030488c597c