Keep tiles on disk, add basket curves, undo the image flip

The flip was my mistake. Measured against the service instead of guessed:
fetching a box and its northern half shows the half matching the top of the
whole, so the images arrive with their first row at the north edge and the
default texture coordinates were right all along. Both the base map and the
orthophoto go back to them.

Imagery of a given piece of ground does not change, so a fetched tile is now
kept under editor/cache/orto and the next session starts from it. Until now
the cache lived only in memory, which meant going back to a slow and flaky
service for ground that had already been fetched once.

Gaps can be fitted with a basket curve as well: the library's compound mode,
with clothoids at both ends and a chain of arcs between them. Arcs can be
added and removed; the closing one takes whatever deflection the earlier
ones leave, so its length is not offered for editing.

application/planpanel.h

@@ -32,6 +32,8 @@ class plan_panel : public ui_panel
 	// methods
 	void render_toolbar();
 	void render_gaps();
+	// parameters of a basket curve: the end clothoids and the chain of arcs between them
+	void render_compound(maj0sted::web::GapFit &Fit);
 	void render_storage();
 	// asks what kind of scenery is being started, and where on the map it sits
 	void render_newmap_dialog();