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.

editor/polandmap.cpp

@@ -26,21 +26,10 @@ struct geo_point
 	double longitude;
 };
 
-// the border walked clockwise from the north-west corner: Baltic coast, the Kaliningrad and
-// Lithuanian borders, the Bug, the Carpathians and Sudetes, then the Nysa and Odra back north.
-// tens of kilometres of error in places - this is a locator sketch, nothing more
-constexpr geo_point kOutline[]{
-    {53.93, 14.27}, {54.05, 14.60}, {54.18, 15.58}, {54.42, 16.41}, {54.58, 16.86}, {54.76, 17.55},
-    {54.79, 18.40}, {54.60, 18.65}, {54.52, 18.55}, {54.35, 18.95}, {54.37, 19.35}, {54.45, 19.64},
-    {54.37, 20.50}, {54.35, 21.50}, {54.36, 22.00}, {54.36, 22.79}, {54.25, 23.20}, {54.15, 23.48},
-    {53.95, 23.90}, {53.20, 23.93}, {52.70, 23.85}, {52.30, 23.60}, {52.07, 23.62}, {51.55, 23.55},
-    {50.87, 24.15}, {50.72, 24.15}, {50.40, 23.70}, {50.28, 23.50}, {49.80, 22.92}, {49.20, 22.68},
-    {49.09, 22.55}, {49.25, 22.00}, {49.42, 21.75}, {49.35, 21.00}, {49.40, 20.50}, {49.20, 20.10},
-    {49.40, 19.80}, {49.40, 19.50}, {49.40, 19.20}, {49.51, 18.85}, {49.75, 18.63}, {49.95, 18.05},
-    {50.10, 17.70}, {50.31, 17.38}, {50.45, 16.90}, {50.20, 16.60}, {50.40, 16.35}, {50.44, 16.25},
-    {50.75, 15.80}, {50.75, 15.55}, {50.85, 15.00}, {50.87, 14.82}, {51.30, 14.95}, {51.60, 14.75},
-    {52.00, 14.70}, {52.35, 14.55}, {52.59, 14.65}, {52.85, 14.40}, {53.10, 14.35}, {53.55, 14.30},
-    {53.75, 14.28}};
+// the country's corners, wide enough to hold all of it: the Odra in the west, the Bug in the east,
+// the Tatras in the south and the coast in the north
+constexpr geo_point kSouthWest{49.00, 14.10};
+constexpr geo_point kNorthEast{54.90, 24.20};
 
 struct named_geo_point
 {
@@ -59,19 +48,17 @@ constexpr named_geo_point kPlaces[]{
 
 } // namespace
 
-std::vector<glm::dvec2> const &poland_outline()
+void poland_extent(glm::dvec2 &Min, glm::dvec2 &Max)
 {
-	static std::vector<glm::dvec2> const outline = []() {
-		std::vector<glm::dvec2> points;
-		points.reserve(std::size(kOutline));
-		for (auto const &point : kOutline)
-		{
-			points.push_back(puwg1992(point.latitude, point.longitude));
-		}
-		return points;
-	}();
+	// the projection is not axis-aligned with the graticule, so the extreme corners come from
+	// projecting all four of them and taking the outermost values
+	auto const sw = puwg1992(kSouthWest.latitude, kSouthWest.longitude);
+	auto const se = puwg1992(kSouthWest.latitude, kNorthEast.longitude);
+	auto const nw = puwg1992(kNorthEast.latitude, kSouthWest.longitude);
+	auto const ne = puwg1992(kNorthEast.latitude, kNorthEast.longitude);
 
-	return outline;
+	Min = {std::min({sw.x, se.x, nw.x, ne.x}), std::min({sw.y, se.y, nw.y, ne.y})};
+	Max = {std::max({sw.x, se.x, nw.x, ne.x}), std::max({sw.y, se.y, nw.y, ne.y})};
 }
 
 std::vector<map_place> const &poland_places()