EEVEE: Add Transparent Render-Pass option

This renderpass pass outputs alpha blender surface
to allow combining them with the opaque passes.

Limitation: This only supports monochromatic opacity.
Colored opacity will show differently than in combined pass.

Pull Request: https://projects.blender.org/blender/blender/pulls/107890

scripts/startup/bl_ui/properties_view_layer.py

@@ -147,6 +147,9 @@ class VIEWLAYER_PT_eevee_layer_passes_effects(ViewLayerButtonsPanel, Panel):
         col.prop(view_layer_eevee, "use_pass_bloom", text="Bloom")
         col.active = scene_eevee.use_bloom
 
+        col = layout.column()
+        col.prop(view_layer_eevee, "use_pass_transparent")
+
 
 class ViewLayerAOVPanel(ViewLayerButtonsPanel, Panel):
     bl_label = "Shader AOV"