adjusts canvas backend size by dpr

Author: Marius Mueller

src/drawing/backend_impl/canvas.rs

@@ -74,8 +74,10 @@ impl DrawingBackend for CanvasBackend {
 
     fn get_size(&self) -> (u32, u32) {
         // Getting just canvas.width gives poor results on HighDPI screens.
-        let rect = self.canvas.get_bounding_client_rect();
-        (rect.width() as u32, rect.height() as u32)
+        let window = window()?;
+        let mut dpr = window.device_pixel_ratio();
+        dpr = if dpr == 0 { 1 } else { dpr };
+        ((canvas.width() as f64 / dpr) as u32, (canvas.height() as f64 / dpr) as u32)
     }
 
     fn ensure_prepared(&mut self) -> Result<(), DrawingErrorKind<CanvasError>> {