Adapt to Core Graphics coordinate system

src/cg.rs

@@ -9,18 +9,25 @@ use core_graphics::geometry::{CGPoint, CGSize, CGRect};
 use core_graphics::image::CGImage;
 use core_graphics::sys;
 
-pub struct CGImpl;
+pub struct CGImpl {
+    view: *mut Object,
+}
 
 impl CGImpl {
     pub unsafe fn new<W: HasRawWindowHandle>(handle: AppKitHandle) -> Result<Self, SoftBufferError<W>> {
         let window = handle.ns_window as *mut Object;
+        let view = handle.ns_view as *mut Object;
         let cls = class!(NSGraphicsContext);
         let graphics_context: *mut Object = msg_send![cls, graphicsContextWithWindow:window];
         if graphics_context.is_null() {
             return Err(SoftBufferError::PlatformError(Some("Graphics context is null".into()), None));
         }
         let _: () = msg_send![cls, setCurrentContext:graphics_context];
-        Ok(Self)
+        Ok(
+            Self {
+                view,
+            }
+        )
     }
 }
 
@@ -47,8 +54,10 @@ impl GraphicsContextImpl for CGImpl {
             false,
             kCGRenderingIntentDefault,
         );
-        let origin = CGPoint { x: 0f64, y: 0f64 };
+        let frame: CGRect = msg_send![self.view, frame];
-        let size = CGSize { width: width as f64, height: height as f64 };
+        // In Core Graphics, (0, 0) is bottom left, not top left
+        let origin = CGPoint { x: 0f64, y: frame.size.height };
+        let size = CGSize { width: width as f64, height: -(height as f64) };
         let rect = CGRect { origin, size };
         context.draw_image(rect, &image);
     }