Expose the cell_size affecting VisibilityNotifier2D precision

Refs: #4803
(cherry picked from commit d49ff7aff7)

doc/classes/ProjectSettings.xml

@@ -1164,6 +1164,9 @@
 		<member name="rendering/vram_compression/import_s3tc" type="bool" setter="" getter="" default="true">
 			If [code]true[/code], the texture importer will import VRAM-compressed textures using the S3 Texture Compression algorithm. This algorithm is only supported on desktop platforms and consoles.
 		</member>
+		<member name="world/2d/cell_size" type="int" setter="" getter="" default="100">
+			Cell size used for the 2D hash grid that [VisibilityNotifier2D] uses.
+		</member>
 	</members>
 	<constants>
 	</constants>

doc/classes/VisibilityEnabler2D.xml

@@ -5,7 +5,7 @@
 	</brief_description>
 	<description>
 		The VisibilityEnabler2D will disable [RigidBody2D], [AnimationPlayer], and other nodes when they are not visible. It will only affect nodes with the same root node as the VisibilityEnabler2D, and the root node itself.
-		[b]Note:[/b] VisibilityEnabler2D uses an approximate heuristic for performance reasons. If you need exact visibility checking, use another method such as adding an [Area2D] node as a child of a [Camera2D] node.
+		[b]Note:[/b] For performance reasons, VisibilityEnabler2D uses an approximate heuristic with precision determined by [member ProjectSettings.world/2d/cell_size]. If you need exact visibility checking, use another method such as adding an [Area2D] node as a child of a [Camera2D] node.
 		[b]Note:[/b] VisibilityEnabler2D will not affect nodes added after scene initialization.
 	</description>
 	<tutorials>

doc/classes/VisibilityNotifier2D.xml

@@ -5,7 +5,7 @@
 	</brief_description>
 	<description>
 		The VisibilityNotifier2D detects when it is visible on the screen. It also notifies when its bounding rectangle enters or exits the screen or a viewport.
-		[b]Note:[/b] VisibilityNotifier2D uses an approximate heuristic for performance reasons. If you need exact visibility checking, use another method such as adding an [Area2D] node as a child of a [Camera2D] node.
+		[b]Note:[/b] For performance reasons, VisibilityNotifier2D uses an approximate heuristic with precision determined by [member ProjectSettings.world/2d/cell_size]. If you need exact visibility checking, use another method such as adding an [Area2D] node as a child of a [Camera2D] node.
 	</description>
 	<tutorials>
 	</tutorials>

scene/resources/world_2d.cpp

@@ -314,7 +314,7 @@ struct SpatialIndexer2D {
 
 		pass = 0;
 		changed = false;
-		cell_size = 100; //should be configurable with GLOBAL_DEF("") i guess
+		cell_size = GLOBAL_DEF("world/2d/cell_size", 100);
 	}
 };