Display: one (hopefully) significant optimization and one minor one

Copying the blit_helper objects when adding them to the drawing buffer was expensive since
blit_helper contains a vector of surfaces. We definitely want to copy that as little as
possible.

Also used std::move when fetching fog/shroud images. This function apparently has little
overhead, but still nice to not make copies here.

Also made get_terrain_images return void (this is just minor cleanup, not an optimization).
No need for it to return a reference to a class member.

src/display.cpp

@@ -1067,15 +1067,15 @@ std::vector<surface> display::get_fog_shroud_images(const map_location& loc, ima
 	std::vector<surface> res;
 
 	for (std::string& name : names) {
-		const surface surf(image::get_image(name, image_type));
+		surface surf(image::get_image(name, image_type));
 		if (surf)
-			res.push_back(surf);
+			res.push_back(std::move(surf));
 	}
 
 	return res;
 }
 
-const std::vector<surface>& display::get_terrain_images(const map_location &loc,
+void display::get_terrain_images(const map_location &loc,
 	const std::string& timeid,
 	TERRAIN_TYPE terrain_type)
 {
@@ -1222,23 +1222,21 @@ const std::vector<surface>& display::get_terrain_images(const map_location &loc,
 			}
 		}
 	}
-
-	return terrain_image_vector_;
 }
 
 void display::drawing_buffer_add(const drawing_layer layer,
 		const map_location& loc, int x, int y, const surface& surf,
 		const SDL_Rect &clip)
 {
-	drawing_buffer_.push_back(blit_helper(layer, loc, x, y, surf, clip));
+	drawing_buffer_.emplace_back(layer, loc, x, y, surf, clip);
 }
 
 void display::drawing_buffer_add(const drawing_layer layer,
 		const map_location& loc, int x, int y,
 		const std::vector<surface> &surf,
 		const SDL_Rect &clip)
 {
-	drawing_buffer_.push_back(blit_helper(layer, loc, x, y, surf, clip));
+	drawing_buffer_.emplace_back(layer, loc, x, y, surf, clip);
 }
 
 // FIXME: temporary method. Group splitting should be made
@@ -2566,13 +2564,13 @@ void display::draw_hex(const map_location& loc) {
 
 	if(!shrouded(loc)) {
 		// unshrouded terrain (the normal case)
-		const std::vector<surface>& images_bg = get_terrain_images(loc, tod.id, BACKGROUND);
-		drawing_buffer_add(LAYER_TERRAIN_BG, loc, xpos, ypos, images_bg);
-		num_images_bg = images_bg.size();
+		get_terrain_images(loc, tod.id, BACKGROUND); // updates terrain_image_vector_
+		drawing_buffer_add(LAYER_TERRAIN_BG, loc, xpos, ypos, terrain_image_vector_);
+		num_images_bg = terrain_image_vector_.size();
 
-		const std::vector<surface>& images_fg = get_terrain_images(loc, tod.id, FOREGROUND);
-		drawing_buffer_add(LAYER_TERRAIN_FG, loc, xpos, ypos, images_fg);
-		num_images_fg = images_fg.size();
+		get_terrain_images(loc, tod.id, FOREGROUND); // updates terrain_image_vector_
+		drawing_buffer_add(LAYER_TERRAIN_FG, loc, xpos, ypos, terrain_image_vector_);
+		num_images_fg = terrain_image_vector_.size();
 
 		// Draw the grid, if that's been enabled
 		if(grid_) {

src/display.hpp

@@ -716,8 +716,7 @@ class display : public filter_context, public video2::draw_layering
 
 	enum TERRAIN_TYPE { BACKGROUND, FOREGROUND};
 
-	// Warning: the returned vector will be invalidated on the next call!
-	const std::vector<surface>& get_terrain_images(const map_location &loc,
+	void get_terrain_images(const map_location &loc,
 					const std::string& timeid,
 					TERRAIN_TYPE terrain_type);
 
@@ -930,6 +929,10 @@ class display : public filter_context, public video2::draw_layering
 	class blit_helper
 	{
 	public:
+		// We don't want to copy this.
+		// It's expensive when done frequently due to the surface vector.
+		blit_helper(const blit_helper&) = delete;
+
 		blit_helper(const drawing_layer layer, const map_location& loc,
 				const int x, const int y, const surface& surf,
 				const SDL_Rect& clip)