mac: Fix layer scale not being updated when contents is updated and window is on a new screen

cc #67

Author: tronical

src/cg.rs

@@ -16,6 +16,7 @@ use std::sync::Arc;
 
 pub struct CGImpl {
     layer: CALayer,
+    window: id,
 }
 
 impl CGImpl {
@@ -27,14 +28,13 @@ impl CGImpl {
             let subview: id = NSView::alloc(nil).initWithFrame_(NSView::frame(view));
             layer.set_contents_gravity(ContentsGravity::TopLeft);
             layer.set_needs_display_on_bounds_change(false);
-            layer.set_contents_scale(window.backingScaleFactor());
             subview.setLayer(layer.id());
             subview.setAutoresizingMask_(NSViewWidthSizable | NSViewHeightSizable);
 
             view.addSubview_(subview); // retains subview (+1) = 2
             let _: () = msg_send![subview, release]; // releases subview (-1) = 1
         }
-        Ok(Self { layer })
+        Ok(Self { layer, window })
     }
 
     pub(crate) unsafe fn set_buffer(&mut self, buffer: &[u32], width: u16, height: u16) {
@@ -62,7 +62,11 @@ impl CGImpl {
         transaction::begin();
         transaction::set_disable_actions(true);
 
-        unsafe { self.layer.set_contents(image.as_ptr() as id) };
+        unsafe {
+            self.layer
+                .set_contents_scale(self.window.backingScaleFactor());
+            self.layer.set_contents(image.as_ptr() as id);
+        };
 
         transaction::commit();
     }