better & more fine grained redraw system (should allow movement)

src/core/text.rs

@@ -41,12 +41,10 @@ impl Widget for Text {
         let (handle, offset) = painter.render_text(&mut self.buf, &self.content, &self.attrs);
         let dims = handle.size();
         let size = offset.size(&handle);
-        let mut region = painter.region().center().expand(size);
+        let mut region = Align::Center.pos().expand(size);
         region.top_left.offset += offset.top_left;
         region.bot_right.offset = region.top_left.offset + dims;
-        painter.draw_texture_at(&handle, region);
-        // TODO: when on_update is added to painter,
-        // reuse TextureHandle
+        painter.draw_texture_within(&handle, region);
     }
 
     fn get_size(&mut self, ctx: &mut SizeCtx) -> Vec2 {