Set bar.pending_manage.output_geometry to true on creation

Without this, if you had a config with no bar, added the bar, then
reloaded the config, its geometry would never be created because of the
early return in Bar.manage(), so nothing would ever get drawn.

src/Context.zig

@@ -216,6 +216,7 @@ pub fn manage(context: *Context) void {
                         log.err("Failed to create bar: {}", .{e});
                         continue;
                     };
+                    output.bar.?.pending_manage.output_geometry = true;
                 }
             }
         }