Default: Add missing 'default_dig_snappy' sound

Some nodes with the snappy group (wool, glass pane, seeds) were missing
sound when dug with a sword.
Adding the sound causes it to be used for glass pane, so define a 'dig'
sound in the 'node sound glass defaults' table, use the footstep sound.
Adding the sound also causes it to be used for seeds which deliberately
have no dig sound, so add an empty sound override for seed dig.

Reduce the gain of the glass footstep sound which was excessively loud.

Add some freesound members to license.txt.

mods/default/README.txt

@@ -252,10 +252,13 @@ Metal sounds:
 Tool breaking sounds added by sofar: CC-BY-3.0
   default_tool_breaks.* - http://www.freesound.org/people/HerbertBoland/sounds/33206/
 
-AGFX
-https://www.freesound.org/people/AGFX/
-(CC BY 3.0):
+AGFX (CC BY 3.0)
+https://www.freesound.org/people/AGFX/packs/1253/
   default_water_footstep.1.ogg
   default_water_footstep.2.ogg
   default_water_footstep.3.ogg
 (default_water_footstep.4.ogg is silent)
+
+blukotek (CC0 1.0)
+https://www.freesound.org/people/blukotek/sounds/251660/
+  default_dig_snappy.ogg

mods/default/functions.lua

@@ -88,7 +88,9 @@ end
 function default.node_sound_glass_defaults(table)
 	table = table or {}
 	table.footstep = table.footstep or
-			{name = "default_glass_footstep", gain = 0.5}
+			{name = "default_glass_footstep", gain = 0.25}
+	table.dig = table.dig or
+			{name = "default_glass_footstep", gain = 0.45}
 	table.dug = table.dug or
 			{name = "default_break_glass", gain = 1.0}
 	default.node_sound_defaults(table)

mods/default/license.txt

@@ -144,6 +144,9 @@ Copyright (C) 2009 cmusounddesign
 Copyright (C) 2010 Tomlija
 Copyright (C) 2010 lsprice
 Copyright (C) 2014 sonictechtonic
+Copyright (C) 2015 yadronoff
+Copyright (C) 2007 HerbertBoland
+Copyright (C) 2006 AGFX
 
 You are free to:
 Share — copy and redistribute the material in any medium or format.

mods/farming/api.lua

@@ -307,6 +307,7 @@ farming.register_plant = function(name, def)
 		},
 		fertility = def.fertility,
 		sounds = default.node_sound_dirt_defaults({
+			dig = {name = "", gain = 0},
 			dug = {name = "default_grass_footstep", gain = 0.2},
 			place = {name = "default_place_node", gain = 0.25},
 		}),