Put Geoportal imagery behind the plan and clear the scenery on a new map

The location picker now shows the real topographic map instead of a
hand-drawn outline, fetched as one WMS image for whatever box is on screen;
the schematic border is gone and only the city list remains, to jump the
view somewhere recognisable. Under the plan itself a georeferenced map gets
the orthophoto, tiled through the layout library's own EPSG:2180 grid and
cache.

What a native build was missing is here: the download (WinHTTP, on worker
threads), the decode and the texture. Finished transfers are handed back on
the thread that drains them, so both the library's cache and the graphics
API stay on one thread. The imagery service turns down browser user agents
and fails the odd request even for a plain one, so requests go out as curl
and are retried a few times.

Starting a new map now drops the loaded scenery and comes back on empty
ground, instead of leaving the previous scenery's contents under a plan that
has nothing to do with them. The reload is deferred to the top of the frame:
taking the editor off the mode stack while it is drawing its own panels
would pull the ground out from under them.

application/editormode.cpp

@@ -609,6 +609,19 @@ void editor_mode::redo_last()
 
 bool editor_mode::update()
 {
+    if (Global.editor_reset_scenery)
+    {
+        // starting a new map: drop whatever scenery is loaded and come back on empty ground. done
+        // here rather than where it was asked for, because taking the editor off the mode stack in
+        // the middle of drawing its own panels would pull the ground out from under them
+        Global.editor_reset_scenery = false;
+        Global.editor_startup = true; // the editor is the base mode from here on, whatever started it
+        Global.SceneryFile = "pusta.scn";
+        Application.pop_mode();
+        Application.push_mode(eu07_application::mode::scenarioloader);
+        return true;
+    }
+
     Timer::UpdateTimers(true);
 
     simulation::State.update_clocks();