Ensure sector polygons are always created when opening a map

For some reason this was only happening on linux, but it would draw the map and init sector polygons before any sides were loaded (so no sectors had any connected side/line info).

Now we don't draw the map until it's fully loaded, and also reset sector polygon info when running the 'building sector polygons' step on map open

src/MapEditor/Renderer/Renderer.cpp

@@ -949,6 +949,13 @@ void Renderer::drawAnimations(gl::draw2d::Context& dc) const
 // -----------------------------------------------------------------------------
 void Renderer::drawMap2d(draw2d::Context& dc) const
 {
+	if (!context_->map().isOpen())
+	{
+		// Map isn't open, just draw the grid
+		drawGrid(dc);
+		return;
+	}
+
 	auto mouse_state = context_->input().mouseState();
 
 	// Draw flats if needed

src/SLADEMap/MapObjectList/SectorList.cpp

@@ -145,6 +145,7 @@ void SectorList::initPolygons() const
 	for (unsigned i = 0; i < count_; ++i)
 	{
 		ui::setSplashProgress(i, count_);
+		objects_[i]->resetPolygon();
 		objects_[i]->polygonVertices();
 	}
 	ui::setSplashProgress(1.0f);

src/SLADEMap/SLADEMap.cpp

@@ -257,6 +257,7 @@ bool SLADEMap::readMap(const MapDesc& map)
 	recomputeSpecials();
 
 	opened_time_ = app::runTimer() + 10;
+	is_open_     = true;
 
 	return ok;
 }
@@ -278,6 +279,8 @@ void SLADEMap::clearMap()
 	for (auto& entry : udmf_extra_entries_)
 		delete entry;
 	udmf_extra_entries_.clear();
+
+	is_open_ = false;
 }
 
 // -----------------------------------------------------------------------------

src/SLADEMap/SLADEMap.h

@@ -37,6 +37,7 @@ class SLADEMap
 	long                       geometryUpdated() const { return geometry_updated_; }
 	long                       thingsUpdated() const { return things_updated_; }
 	const MapObjectCollection& mapData() const { return data_; }
+	bool                       isOpen() const { return is_open_; }
 
 	void setGeometryUpdated();
 	void setThingsUpdated();
@@ -155,6 +156,7 @@ class SLADEMap
 	MapFormat               current_format_;
 	long                    opened_time_ = 0;
 	unique_ptr<MapSpecials> map_specials_;
+	bool                    is_open_ = false;
 
 	vector<ArchiveEntry*> udmf_extra_entries_; // UDMF Extras