Fixed crash when starting the editor (fixes #2816)

If no display_context was passed to the display ctor (such as is the case in the editor),
the terrain_builder's gamemap pointer would be null. Really not sure why this didn't cause
a crash n the 1.14 branch, but oh well.

I just removed the ctor call to rebuild_all() since it's not really needed. The map is
rebuilt when change_display_context is called anyway, which both the editor_controller and
play_controller do.