Restore bounded_add()

Partial revert of commit ecbb15e1c6.

Replacing bounded_add() with std::clamp() in these contexts disables
time-of-day bonuses, since ToD bonuses (the base value in these functions)
are almost always outside the range set in terrain properties.

src/terrain/terrain.hpp

@@ -55,7 +55,7 @@ public:
 	/// gives a @a base bonus.
 	int light_bonus(int base) const
 	{
-		return utils::clamp(base + light_modification_, min_light_, max_light_);
+		return bounded_add(base, light_modification_, max_light_, min_light_);
 	}
 
 	int unit_height_adjust() const { return height_adjust_; }

src/tod_manager.cpp

@@ -250,7 +250,7 @@ const time_of_day tod_manager::get_illuminated_time_of_day(const unit_map & unit
 		int best_result = terrain_light;
 		const int base_light = terrain_light + (net_darker ? most_add : most_sub);
 		for ( size_t i = 0; i != mod_list.size(); ++i ) {
-			int result = utils::clamp(base_light + mod_list[i], min_list[i], max_list[i]);
+			int result = bounded_add( base_light, mod_list[i], max_list[i], min_list[i] );
 
 			if ( net_darker  &&  result < best_result )
 				best_result = result;

src/utils/math.hpp

@@ -39,6 +39,20 @@ inline int div100rounded(int num) {
 	return (num < 0) ? -(((-num) + 50) / 100) : (num + 50) / 100;
 }
 
+/**
+ * Returns base + increment, but will not increase base above max_sum, nor
+ * decrease it below min_sum.
+ * (If base is already below the applicable limit, base will be returned.)
+ */
+inline int bounded_add(int base, int increment, int max_sum, int min_sum = 0)
+{
+	if(increment >= 0) {
+		return std::min(base + increment, std::max(base, max_sum));
+	} else {
+		return std::max(base + increment, std::min(base, min_sum));
+	}
+}
+
 /**
  *  round (base_damage * bonus / divisor) to the closest integer,
  *  but up or down towards base_damage