Eevee: Add irradiance smoothing

This is a parameter that will make the interpolation between irradiance
cells of a same Irradiance Volume smoother, reducing the weight of the
light leaking correction factors.

It is usefull in some cases to avoid harsh lighting transition that can
happen when a sample point it near a surface.

release/scripts/startup/bl_ui/properties_render.py

@@ -446,6 +446,8 @@ class RENDER_PT_eevee_indirect_lighting(RenderButtonsPanel, Panel):
         col.prop(props, "gi_diffuse_bounces")
         col.prop(props, "gi_cubemap_resolution")
         col.prop(props, "gi_visibility_resolution", text="Diffuse Occlusion")
+        col.prop(props, "gi_irradiance_smoothing")
+
 
 class RENDER_PT_eevee_indirect_lighting_display(RenderButtonsPanel, Panel):
     bl_label = "Display"