Fix texture without minetest_game installed

mod.conf

@@ -1,2 +1,2 @@
 name = snippets
-optional_depends = cloaking, fs51
+optional_depends = cloaking, default, fs51

nodes.lua

@@ -2,10 +2,17 @@
 -- Buttons that run snippets
 --
 
+-- Use steel block texture in minetest_game
+local bg
+if minetest.registered_nodes['default:steelblock'] then
+    bg = 'default_steel_block.png'
+else
+    bg = '[combine:1x1^[noalpha^[colorize:#aaa'
+end
+
 minetest.register_node('snippets:button', {
     description = 'Snippets button',
-    tiles = {'default_steel_block.png', 'default_steel_block.png',
-        'default_steel_block.png^snippets_button.png'},
+    tiles = {bg, bg, bg .. '^snippets_button.png'},
     groups = {cracky = 2, not_in_creative_inventory = 1},
 
     on_construct = function(pos)