Fix #122847: Show Transparent Render Pass

EEVEE has a transparent render pass that renders materials where
the render mode is set to blended. This was introduced when EEVEE-Next
was still in development, but was never fully backported.

This PR adds the missing pieces.

Pull Request: https://projects.blender.org/blender/blender/pulls/123298

scripts/startup/bl_ui/properties_view_layer.py

@@ -184,6 +184,7 @@ class VIEWLAYER_PT_eevee_next_layer_passes_light(ViewLayerButtonsPanel, Panel):
         col.prop(view_layer, "use_pass_environment")
         col.prop(view_layer, "use_pass_shadow")
         col.prop(view_layer, "use_pass_ambient_occlusion", text="Ambient Occlusion")
+        col.prop(view_layer_eevee, "use_pass_transparent", text="Transparent")
 
         col = layout.column()
         col.active = view_layer.use_pass_ambient_occlusion

source/blender/draw/engines/eevee_next/eevee_film.hh

@@ -155,7 +155,10 @@ class Film {
   static bool pass_is_float3(eViewLayerEEVEEPassType pass_type)
   {
     return pass_storage_type(pass_type) == PASS_STORAGE_COLOR &&
-           !ELEM(pass_type, EEVEE_RENDER_PASS_COMBINED, EEVEE_RENDER_PASS_VECTOR);
+           !ELEM(pass_type,
+                 EEVEE_RENDER_PASS_COMBINED,
+                 EEVEE_RENDER_PASS_VECTOR,
+                 EEVEE_RENDER_PASS_TRANSPARENT);
   }
 
   /* Returns layer offset in the accumulation texture. -1 if the pass is not enabled. */

source/blender/draw/engines/eevee_next/eevee_instance.cc

@@ -611,6 +611,7 @@ void Instance::update_passes(RenderEngine *engine, Scene *scene, ViewLayer *view
   CHECK_PASS_LEGACY(ENVIRONMENT, SOCK_RGBA, 3, "RGB");
   CHECK_PASS_LEGACY(SHADOW, SOCK_RGBA, 3, "RGB");
   CHECK_PASS_LEGACY(AO, SOCK_RGBA, 3, "RGB");
+  CHECK_PASS_EEVEE(TRANSPARENT, SOCK_RGBA, 4, "RGBA");
 
   LISTBASE_FOREACH (ViewLayerAOV *, aov, &view_layer->aovs) {
     if ((aov->flag & AOV_CONFLICT) != 0) {