Fix use of Clip inside a scrolled Flickable with the GL backend

We're using `Canvas::intersect_scissor` to combine the viewport clip of
the flickable with any child `Clip` element, for example. Unfortunately
`intersect_scissor` has a bug and this patch works around it by doing
the scissor intersection manually. This works because we don't apply a
global transform on the canvas -- all scaling and translation is done
locally in the draw functions.

sixtyfps_runtime/rendering_backends/gl/lib.rs

@@ -589,7 +589,12 @@ impl GLRenderer {
             canvas.clear_rect(0, 0, size.width, size.height, clear_color.into());
         }
 
-        GLItemRenderer { shared_data: self.shared_data.clone(), scale_factor }
+        GLItemRenderer {
+            shared_data: self.shared_data.clone(),
+            scale_factor,
+            saved_scissor_rects: Vec::new(),
+            current_scissor_rect: None,
+        }
     }
 
     /// Complete the item rendering by calling this function. This will typically flush any remaining/pending
@@ -649,6 +654,8 @@ impl GLRenderer {
 pub struct GLItemRenderer {
     shared_data: Rc<GLRendererData>,
     scale_factor: f32,
+    saved_scissor_rects: Vec<Rect>,
+    current_scissor_rect: Option<Rect>,
 }
 
 fn rect_to_path(r: Rect) -> femtovg::Path {
@@ -1102,19 +1109,30 @@ impl ItemRenderer for GLItemRenderer {
 
     fn combine_clip(&mut self, pos: Point, rect: Rect) {
         let clip_rect = rect.translate([pos.x, pos.y].into());
-        self.shared_data.canvas.borrow_mut().intersect_scissor(
-            clip_rect.min_x(),
-            clip_rect.min_y(),
-            clip_rect.width(),
-            clip_rect.height(),
+        // FIXME: Go back to using Canvas::intersect_scissor when https://github.com/femtovg/femtovg/pull/29 is merged
+        let final_clip_rect = if let Some(existing_clip) = self.current_scissor_rect {
+            existing_clip.intersection(&clip_rect).unwrap_or_default()
+        } else {
+            clip_rect
+        };
+        self.shared_data.canvas.borrow_mut().scissor(
+            final_clip_rect.min_x(),
+            final_clip_rect.min_y(),
+            final_clip_rect.width(),
+            final_clip_rect.height(),
         );
+        self.current_scissor_rect = Some(final_clip_rect);
     }
 
     fn save_state(&mut self) {
+        if let Some(scissor_rect) = self.current_scissor_rect {
+            self.saved_scissor_rects.push(scissor_rect);
+        }
         self.shared_data.canvas.borrow_mut().save();
     }
 
     fn restore_state(&mut self) {
+        self.current_scissor_rect = self.saved_scissor_rects.pop();
         self.shared_data.canvas.borrow_mut().restore();
     }