Implement Widget::mouse_interaction for PaneGrid

... and fix rendering of drag interaction in `PaneGrid` by
introducing an explicit `with_translation` method to `Renderer`
and simplifying the `with_layer` and `Clip` primitive.

graphics/src/primitive.rs

@@ -66,8 +66,6 @@ pub enum Primitive {
     Clip {
         /// The bounds of the clip
         bounds: Rectangle,
-        /// The offset transformation of the clip
-        offset: Vector<u32>,
         /// The content of the clip
         content: Box<Primitive>,
     },