Fix a set_sky crash

We can't use push incremental changes to set_sky, we need to
push the whole sky definition each time.

docs/issues-code.txt

@@ -24,6 +24,13 @@ ISSUES-CODE: Issues related to code quality and APIs
 	- Break up hints into each individual mod.
 	- Add shortcuts for hint registrations, e.g. integrated with recipes.
 
+- Transactional api ref localization
+	- For certain large computations, like an optic processing, it may be
+	  worthwhile to capture some often-repeated functions like minetest.get_node
+	  as locals.
+	- May need to test performance on a non-JIT build, where this is expected
+	  to have a larger impact, since JIT probably optimizes the difference away.
+
 - Continue to chisel away at ABM perf issues.
 	- Simplify or optimize existing ABMs
 	- Convert things to DNTs.

mods/nc_api_active/playerstep.lua

@@ -120,8 +120,9 @@ local function step_player(player, dtime)
 	if mismatch(data.animation, orig_anim) then
 		player:set_animation(unpack(data.animation))
 	end
-	local sky = setdelta(data.sky, orig_sky)
-	if sky then setsky(player, sky) end
+	if mismatch(data.sky, orig_sky) then
+		setsky(player, data.sky)
+	end
 	if mismatch(data.daynight, orig_daynight) then
 		player:override_day_night_ratio(data.daynight)
 	end