EEVEE: Lookdev: Add support for partially blurred background

This is using the GGX probe as background. This has the drawback of
having the resolution choosed in the indirect lighting setting.
The blurring is not really high-quality.

The pros is that it has a simple implementation and is fast to evaluate.

This patch also fades the background alpha to make overlay engine draw the
default background color in the correct color space. Removing one colorspace
hack.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D6895

source/blender/blenloader/intern/versioning_280.c

@@ -4498,5 +4498,19 @@ void blo_do_versions_280(FileData *fd, Library *UNUSED(lib), Main *bmain)
         }
       }
     }
+
+    /* Add Lookdev blur property. */
+    if (!DNA_struct_elem_find(fd->filesdna, "View3DShading", "float", "studiolight_blur")) {
+      for (bScreen *screen = bmain->screens.first; screen; screen = screen->id.next) {
+        for (ScrArea *sa = screen->areabase.first; sa; sa = sa->next) {
+          for (SpaceLink *sl = sa->spacedata.first; sl; sl = sl->next) {
+            if (sl->spacetype == SPACE_VIEW3D) {
+              View3D *v3d = (View3D *)sl;
+              v3d->shading.studiolight_blur = 0.5f;
+            }
+          }
+        }
+      }
+    }
   }
 }