Implement basic layer merging for graphics::layer::Stack

wgpu/src/image/null.rs

@@ -11,4 +11,6 @@ impl Batch {
     pub fn is_empty(&self) -> bool {
         true
     }
+
+    pub fn append(&mut self, _batch: &mut Self) {}
 }

wgpu/src/layer.rs

@@ -268,6 +268,10 @@ impl graphics::Layer for Layer {
         }
     }
 
+    fn bounds(&self) -> Rectangle {
+        self.bounds
+    }
+
     fn flush(&mut self) {
         self.flush_meshes();
         self.flush_text();
@@ -288,6 +292,34 @@ impl graphics::Layer for Layer {
         self.pending_meshes.clear();
         self.pending_text.clear();
     }
+
+    fn level(&self) -> usize {
+        if !self.text.is_empty() {
+            return 4;
+        }
+
+        if !self.images.is_empty() {
+            return 3;
+        }
+
+        if !self.primitives.is_empty() {
+            return 2;
+        }
+
+        if !self.triangles.is_empty() {
+            return 1;
+        }
+
+        0
+    }
+
+    fn merge(&mut self, layer: &mut Self) {
+        self.quads.append(&mut layer.quads);
+        self.triangles.append(&mut layer.triangles);
+        self.primitives.append(&mut layer.primitives);
+        self.images.append(&mut layer.images);
+        self.text.append(&mut layer.text);
+    }
 }
 
 impl Default for Layer {

wgpu/src/quad.rs

@@ -304,6 +304,12 @@ impl Batch {
         self.gradients.clear();
         self.order.clear();
     }
+
+    pub fn append(&mut self, batch: &mut Batch) {
+        self.solids.append(&mut batch.solids);
+        self.gradients.append(&mut batch.gradients);
+        self.order.append(&mut batch.order);
+    }
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]