Eevee: Implement Overscan option

This option make the internal render size larger than the output size in
order to minimize screenspace effects disapearing at the render edges.

The overscan size added around the render is the maximum dimension
multiplied by the overscan percentage.

source/blender/blenloader/intern/versioning_280.c

@@ -2226,4 +2226,12 @@ void blo_do_versions_280(FileData *fd, Library *UNUSED(lib), Main *bmain)
 			        | OB_EMPTY_IMAGE_VISIBLE_ORTHOGRAPHIC);
 		}
 	}
+
+	{
+		if (!DNA_struct_elem_find(fd->filesdna, "SceneEEVEE", "float", "overscan")) {
+			for (Scene *scene = bmain->scene.first; scene; scene = scene->id.next) {
+				scene->eevee.overscan = 3.0f;
+			}
+		}
+	}
 }