Add 2 or 3 lines of code allowing to use a 2-hex Attack Direction Indicator

I also commit some placeholder images, to show how it works and hoping
that someone will improve them

I use a not-too-much-visible quick-gimp, in case it stay longer than
expected, but still a-bit-ugly to motivate some artist to improve them
(and because i can't do better in less than 1 day of gimp-work :-/ )

src/game_display.cpp

@@ -384,7 +384,13 @@ void game_display::draw(bool update,bool force)
 
 			// draw the attack direction indicator
 			if(*it == attack_indicator_to_) {
-				tile_stack_append(image::get_image(attack_indicator_image_, image::UNMASKED));
+				const std::string dir = gamemap::location::write_direction(
+					attack_indicator_to_.get_relative_dir(attack_indicator_from_));
+				tile_stack_append(image::get_image("misc/attack-from-" + dir + ".png", image::UNMASKED));
+			} else if (*it == attack_indicator_from_) {
+				const std::string dir = gamemap::location::write_direction(
+					attack_indicator_from_.get_relative_dir(attack_indicator_to_));
+				tile_stack_append(image::get_image("misc/attack-to-" + dir + ".png", image::UNMASKED));
 			}
 
 			// apply shroud anf fog
@@ -1104,8 +1110,6 @@ void game_display::set_attack_indicator(const gamemap::location& from, const gam
 
 		attack_indicator_from_ = from;
 		attack_indicator_to_ = to;
-		std::string dir = gamemap::location::write_direction(to.get_relative_dir(from));
-		attack_indicator_image_ = "misc/attack-from-" + dir + ".png";
 
 		invalidate(attack_indicator_from_);
 		invalidate(attack_indicator_to_);

src/game_display.hpp

@@ -193,10 +193,9 @@ private:
 	unit *temp_unit_;
 	gamemap::location temp_unit_loc_;
 
-	//location and direction of the attack direction indicator
+	//locations of the attack direction indicator's parts
 	gamemap::location attack_indicator_to_;
 	gamemap::location attack_indicator_from_;
-	std::string attack_indicator_image_;
 
 	//function which finds the start and end rows on the energy bar image
 	//where white pixels are substituted for the colour of the energy