Tiny optimization of screen update.

Update only once by hex instead of once by tblit. The tblit update had
incorrect size and fixing this will slow us.

Also, only the drawer can know how to efficently update (without
relying on our rectangle merging algo)

src/display.cpp

@@ -866,8 +866,8 @@ void display::drawing_buffer_commit()
 			SDL_Rect *srcrectArg = (srcrect.x | srcrect.y | srcrect.w | srcrect.h)
 				? &srcrect : NULL;
 			SDL_BlitSurface(surf, srcrectArg, screen, &dstrect);
+			//NOTE: the screen part should already be marked as 'to update'
 		}
-		update_rect(blit.x(), blit.y(), zoom_, zoom_);
 	}
 	drawing_buffer_clear();
 }
@@ -2003,6 +2003,9 @@ void display::draw_invalidated() {
 	foreach (map_location loc, invalidated_) {
 		int xpos = get_location_x(loc);
 		int ypos = get_location_y(loc);
+
+		update_rect(xpos, ypos, zoom_, zoom_);
+
 		const bool on_map = get_map().on_board(loc);
 		SDL_Rect hex_rect = create_rect(xpos, ypos, zoom_, zoom_);
 		if(!rects_overlap(hex_rect,clip_rect)) {

src/display.hpp

@@ -790,7 +790,7 @@ protected:
 public:
 
 	/**
-	 * Add an item to the drawing buffer.
+	 * Add an item to the drawing buffer. You need to update screen on affected area
 	 *
 	 * @param layer              The layer to draw on.
 	 * @param loc                The hex the image belongs to, needed for the