Added fadein functionality for bgm

This lets the user fadein bgm after using a normal "play bgm" command in
the event editor.  Just simply call "Script.fadein_bgm(target, speed)"
with the target being the target vol % that you want the fade in to
eventually reach and the speed being how fast you want it to reach it (1
- slowest, 100-instant)

scripts/Game_System.rb

@@ -20,6 +20,7 @@ class Game_System
   attr_accessor :message_frame            # text option: window frame
   attr_accessor :save_count               # save count
   attr_accessor :magic_number             # magic number
+  attr_accessor :playing_bgm
   #--------------------------------------------------------------------------
   # * Object Initialization
   #--------------------------------------------------------------------------

scripts/Game_Temp.rb

@@ -9,6 +9,9 @@ class Game_Temp
   #--------------------------------------------------------------------------
   # * Public Instance Variables
   #--------------------------------------------------------------------------
+  attr_accessor :target_bgm_vol_level     # volume we want to reach when fading in music
+  attr_accessor :bgm_fadein_speed         # speed at which we will reach target bgm vol_level
+  attr_accessor :bgm_fadein_timer         # used so we aren't changing the volume every frame
   attr_accessor :map_bgm                  # map music (for battle memory)
   attr_accessor :message_text             # message text
   attr_accessor :message_face             # face graphic
@@ -62,6 +65,9 @@ class Game_Temp
   # * Object Initialization
   #--------------------------------------------------------------------------
   def initialize
+    @target_bgm_vol_level = -1
+    @bgm_fadein_speed = 0
+    @bgm_fadein_timer = 0
     @map_bgm = nil
     @message_text = nil
     @message_face = nil
@@ -111,4 +117,20 @@ class Game_Temp
     @footstep_sfx = nil
     @filmsprite = nil
   end
+
+  def bgm_fadein(game_system)
+    if @target_bgm_vol_level > 0
+      @bgm_fadein_timer += 1
+      if @bgm_fadein_timer > 14
+        @bgm_fadein_timer = 0
+        game_system.playing_bgm.volume += @bgm_fadein_speed
+        if game_system.playing_bgm.volume >= @target_bgm_vol_level
+          game_system.playing_bgm.volume = @target_bgm_vol_level
+          @target_bgm_vol_level = -1
+          @bgm_fadein_speed = 1
+        end
+        game_system.bgm_play(game_system.playing_bgm)
+      end
+    end
+  end
 end

scripts/Scene_Map.rb

@@ -94,6 +94,7 @@ class Scene_Map
     end
     # Loop
     loop do
+      $game_temp.bgm_fadein($game_system)
       # Update map, interpreter, and player order
       # (this update order is important for when conditions are fulfilled
       # to run any event, and the player isn't provided the opportunity to

scripts/Script.rb

@@ -117,6 +117,11 @@ module Script
     $game_map.display_x = x
     $game_map.display_y = y
   end
+
+  def self.fadein_bgm(target, speed)
+    $game_temp.target_bgm_vol_level = target
+    $game_temp.bgm_fadein_speed = speed
+  end
 end
 
 def has_lightbulb?