Redraw invalidated hexes once, not twice

This halves the CPU usage cost of animations, which is useful especially in
water-heavy scenarios such as Dead Water.

I was worried about the possibility of regressions, but I didn't find any
in my playtesting: moved and killed units disappear from their hexes like
they should.

changelog

@@ -52,6 +52,8 @@ Version 1.13.8+dev:
    * Fixed delay or clients gettings stuck when starting a mp game (Bug #1674)
  * Performance:
    * Rewrote the FPS cap implementation. This greatly improves smoothness ingame.
+   * Each invalidated hex is now only redrawn once, not twice. This almost halves CPU
+     usage ingame.
    * Implemented GUI canvas caching. It speeds up multiple areas, but especially
      the story screen.
  * Units:

players_changelog

@@ -35,6 +35,8 @@ Version 1.13.8+dev:
 
  * Performance:
    * Rewrote the FPS cap implementation. This greatly improves smoothness ingame.
+   * Each invalidated hex is now only redrawn once, not twice. This almost halves CPU
+     usage ingame.
    * Implemented GUI canvas caching. It speeds up multiple areas, but especially
      the story screen.
 

src/display.cpp

@@ -189,7 +189,6 @@ display::display(const display_context * dc, CVideo& video, std::weak_ptr<wb::ma
 	menu_buttons_(),
 	action_buttons_(),
 	invalidated_(),
-	previous_invalidated_(),
 	mouseover_hex_overlay_(nullptr),
 	tod_hex_mask1(nullptr),
 	tod_hex_mask2(nullptr),
@@ -2441,17 +2440,7 @@ void display::draw(bool update,bool force) {
 	pre_draw();
 	// invalidate all that needs to be invalidated
 	invalidate_animations();
-	// at this stage we have everything that needs to be invalidated for this redraw
-	// save it as the previous invalidated, and merge with the previous invalidated_
-	// we merge with the previous redraw because if a hex had a unit last redraw but
-	// not this one, nobody will tell us to redraw (cleanup)
-	previous_invalidated_.swap(invalidated_);
-	invalidated_.insert(previous_invalidated_.begin(),previous_invalidated_.end());
-	// these new invalidations cannot cause any propagation because
-	// if a hex was invalidated last turn but not this turn, then
-	// * case of no unit in neighbor hex=> no propagation
-	// * case of unit in hex but was there last turn=>its hexes are invalidated too
-	// * case of unit in hex not there last turn => it moved, so was invalidated previously
+
 	if(!get_map().empty()) {
 		//int simulate_delay = 0;
 

src/display.hpp

@@ -750,7 +750,6 @@ protected:
 	std::map<std::string, config> reports_;
 	std::vector<std::shared_ptr<gui::button>> menu_buttons_, action_buttons_;
 	std::set<map_location> invalidated_;
-	std::set<map_location> previous_invalidated_;
 	surface mouseover_hex_overlay_;
 	// If we're transitioning from one time of day to the next,
 	// then we will use these two masks on top of all hexes when we blit.