Web: Disable raycast module by default (no occlusion culling)

This means no CPU occlusion culling (and not compiling Embree), unless
you compile custom export templates with `module_raycast_enabled=yes`.

This reduces the memory footprint significantly, and binary size.

Fixes #70621.

Co-authored-by: Hugo Locurcio <hugo.locurcio@hugo.pro>

doc/classes/OccluderInstance3D.xml

@@ -9,6 +9,7 @@
 		The occlusion culling system works by rendering the occluders on the CPU in parallel using [url=https://www.embree.org/]Embree[/url], drawing the result to a low-resolution buffer then using this to cull 3D nodes individually. In the 3D editor, you can preview the occlusion culling buffer by choosing [b]Perspective > Debug Advanced... > Occlusion Culling Buffer[/b] in the top-left corner of the 3D viewport. The occlusion culling buffer quality can be adjusted in the Project Settings.
 		[b]Baking:[/b] Select an [OccluderInstance3D] node, then use the [b]Bake Occluders[/b] button at the top of the 3D editor. Only opaque materials will be taken into account; transparent materials (alpha-blended or alpha-tested) will be ignored by the occluder generation.
 		[b]Note:[/b] Occlusion culling is only effective if [member ProjectSettings.rendering/occlusion_culling/use_occlusion_culling] is [code]true[/code]. Enabling occlusion culling has a cost on the CPU. Only enable occlusion culling if you actually plan to use it. Large open scenes with few or no objects blocking the view will generally not benefit much from occlusion culling. Large open scenes generally benefit more from mesh LOD and visibility ranges ([member GeometryInstance3D.visibility_range_begin] and [member GeometryInstance3D.visibility_range_end]) compared to occlusion culling.
+		[b]Note:[/b] Due to memory constraints, occlusion culling is not supported by default in Web export templates. It can be enabled by compiling custom Web export templates with [code]module_raycast_enabled=yes[/code].
 	</description>
 	<tutorials>
 		<link title="Occlusion culling">$DOCS_URL/tutorials/3d/occlusion_culling.html</link>

doc/classes/ProjectSettings.xml

@@ -2518,6 +2518,7 @@
 		<member name="rendering/occlusion_culling/use_occlusion_culling" type="bool" setter="" getter="" default="false">
 			If [code]true[/code], [OccluderInstance3D] nodes will be usable for occlusion culling in 3D in the root viewport. In custom viewports, [member Viewport.use_occlusion_culling] must be set to [code]true[/code] instead.
 			[b]Note:[/b] Enabling occlusion culling has a cost on the CPU. Only enable occlusion culling if you actually plan to use it. Large open scenes with few or no objects blocking the view will generally not benefit much from occlusion culling. Large open scenes generally benefit more from mesh LOD and visibility ranges ([member GeometryInstance3D.visibility_range_begin] and [member GeometryInstance3D.visibility_range_end]) compared to occlusion culling.
+			[b]Note:[/b] Due to memory constraints, occlusion culling is not supported by default in Web export templates. It can be enabled by compiling custom Web export templates with [code]module_raycast_enabled=yes[/code].
 		</member>
 		<member name="rendering/reflections/reflection_atlas/reflection_count" type="int" setter="" getter="" default="64">
 			Number of cubemaps to store in the reflection atlas. The number of [ReflectionProbe]s in a scene will be limited by this amount. A higher number requires more VRAM.

doc/classes/Viewport.xml

@@ -364,6 +364,7 @@
 		<member name="use_occlusion_culling" type="bool" setter="set_use_occlusion_culling" getter="is_using_occlusion_culling" default="false">
 			If [code]true[/code], [OccluderInstance3D] nodes will be usable for occlusion culling in 3D for this viewport. For the root viewport, [member ProjectSettings.rendering/occlusion_culling/use_occlusion_culling] must be set to [code]true[/code] instead.
 			[b]Note:[/b] Enabling occlusion culling has a cost on the CPU. Only enable occlusion culling if you actually plan to use it, and think whether your scene can actually benefit from occlusion culling. Large, open scenes with few or no objects blocking the view will generally not benefit much from occlusion culling. Large open scenes generally benefit more from mesh LOD and visibility ranges ([member GeometryInstance3D.visibility_range_begin] and [member GeometryInstance3D.visibility_range_end]) compared to occlusion culling.
+			[b]Note:[/b] Due to memory constraints, occlusion culling is not supported by default in Web export templates. It can be enabled by compiling custom Web export templates with [code]module_raycast_enabled=yes[/code].
 		</member>
 		<member name="use_taa" type="bool" setter="set_use_taa" getter="is_using_taa" default="false">
 			Enables Temporal Anti-Aliasing for this viewport. TAA works by jittering the camera and accumulating the images of the last rendered frames, motion vector rendering is used to account for camera and object motion.

platform/web/detect.py

@@ -60,6 +60,8 @@ def get_flags():
         ("target", "template_debug"),
         ("builtin_pcre2_with_jit", False),
         ("vulkan", False),
+        # Embree is heavy and requires too much memory (GH-70621).
+        ("module_raycast_enabled", False),
         # Use -Os to prioritize optimizing for reduced file size. This is
         # particularly valuable for the web platform because it directly
         # decreases download time.