Sky's edge

- Fix darkness when collecting light sections from high in the air
- Add dummy data layer impl to return a constant
- Empty sky section returns 15
- Empty block section returns 0
- Copy vanilla's logic for determining which data layer to use for a
  given position's sky light value
- While I'm at it, retrieve light section storage via an accessor to
  avoid allocating SectionPos objects
- Improve lut debug view

common/src/backend/java/dev/engine_room/flywheel/backend/SkyLightSectionStorageExtension.java

@@ -0,0 +1,9 @@
+package dev.engine_room.flywheel.backend;
+
+import org.jetbrains.annotations.Nullable;
+
+import net.minecraft.world.level.chunk.DataLayer;
+
+public interface SkyLightSectionStorageExtension {
+	@Nullable DataLayer flywheel$skyDataLayer(long section);
+}