initial mask impl

src/core/mask.rs

@@ -0,0 +1,12 @@
+use crate::prelude::*;
+
+pub struct Masked {
+    pub inner: WidgetId,
+}
+
+impl Widget for Masked {
+    fn draw(&mut self, painter: &mut Painter) {
+        painter.set_mask(painter.region());
+        painter.widget(&self.inner);
+    }
+}