Fix volumetric fog not rendering at densities lower than or equal to 0.001

This allows volumetric fog to render with the lowest density that
can be specified in the inspector (0.0001).

doc/classes/Environment.xml

@@ -301,6 +301,7 @@
 		<member name="volumetric_fog_density" type="float" setter="set_volumetric_fog_density" getter="get_volumetric_fog_density" default="0.05">
 			The base [i]exponential[/i] density of the volumetric fog. Set this to the lowest density you want to have globally. [FogVolume]s can be used to add to or subtract from this density in specific areas. Fog rendering is exponential as in real life.
 			A value of [code]0.0[/code] disables global volumetric fog while allowing [FogVolume]s to display volumetric fog in specific areas.
+			To make volumetric fog work as a volumetric [i]lighting[/i] solution, set [member volumetric_fog_density] to the lowest non-zero value ([code]0.0001[/code]) then increase lights' [member Light3D.light_volumetric_fog_energy] to values between [code]10000[/code] and [code]100000[/code] to compensate for the very low density.
 		</member>
 		<member name="volumetric_fog_detail_spread" type="float" setter="set_volumetric_fog_detail_spread" getter="get_volumetric_fog_detail_spread" default="2.0">
 			The distribution of size down the length of the froxel buffer. A higher value compresses the froxels closer to the camera and places more detail closer to the camera.

doc/classes/FogMaterial.xml

@@ -15,6 +15,7 @@
 		</member>
 		<member name="density" type="float" setter="set_density" getter="get_density" default="1.0">
 			The density of the [FogVolume]. Denser objects are more opaque, but may suffer from under-sampling artifacts that look like stripes. Negative values can be used to subtract fog from other [FogVolume]s or global volumetric fog.
+			[b]Note:[/b] Due to limited precision, [member density] values between [code]-0.001[/code] and [code]0.001[/code] (exclusive) act like [code]0.0[/code]. This does not apply to [member Environment.volumetric_fog_density].
 		</member>
 		<member name="density_texture" type="Texture3D" setter="set_density_texture" getter="get_density_texture">
 			The 3D texture that is used to scale the [member density] of the [FogVolume]. This can be used to vary fog density within the [FogVolume] with any kind of static pattern. For animated effects, consider using a custom [url=$DOCS_URL/tutorials/shaders/shader_reference/fog_shader.html]fog shader[/url].

servers/rendering/renderer_rd/shaders/environment/volumetric_fog_process.glsl

@@ -381,7 +381,7 @@ void main() {
 	float cell_depth_size = abs(view_pos.z - get_depth_at_pos(fog_cell_size.z, pos.z + 1));
 	//compute directional lights
 
-	if (total_density > 0.001) {
+	if (total_density > 0.00005) {
 		for (uint i = 0; i < params.directional_light_count; i++) {
 			if (directional_lights.data[i].volumetric_fog_energy > 0.001) {
 				vec3 shadow_attenuation = vec3(1.0);