add "random return" CPU limiter to active queue playback

set `biome_lib_queue_run_ratio = <1 to 100>` to use
default 100 (basically, a percentage of max runtime).

API.txt

@@ -481,6 +481,12 @@ Set this to true if you want the mod to spam your console with debug info :-)
 
 	plantlife_debug = false
 
+To slow down the playback of the queue (e.g. for really slow machines where
+the 0.2 second max limiter isn't enough), set:
+
+	biome_lib_queue_run_ratio = <some value 1 to 100>
+
+Default is 100 (basically percent of maximum runtime)
 
 ======================
 Fertile Ground Mapping

init.lua

@@ -31,6 +31,8 @@ biome_lib.modpath = minetest.get_modpath("biome_lib")
 
 biome_lib.total_no_aircheck_calls = 0
 
+biome_lib.queue_run_ratio = tonumber(minetest.settings:get("biome_lib_queue_run_ratio")) or 100
+
 -- Boilerplate to support localized strings if intllib mod is installed.
 local S
 if minetest.get_modpath("intllib") then
@@ -427,8 +429,9 @@ end)
 -- "Play" them back, populating them with new stuff in the process
 
 minetest.register_globalstep(function(dtime)
-	if dtime < 0.2 and    -- don't attempt to populate if lag is already too high
-	  (#biome_lib.blocklist_aircheck > 0 or #biome_lib.blocklist_no_aircheck > 0) then
+	if dtime < 0.2    -- don't attempt to populate if lag is already too high
+	  and math.random(100) <= biome_lib.queue_run_ratio
+	  and (#biome_lib.blocklist_aircheck > 0 or #biome_lib.blocklist_no_aircheck > 0) then
 		biome_lib.globalstep_start_time = minetest.get_us_time()
 		biome_lib.globalstep_runtime = 0
 		while (#biome_lib.blocklist_aircheck > 0 or #biome_lib.blocklist_no_aircheck > 0)