[librsvg] ViewParams: keep a Weak reference to the view box stack, not a strong one

[Federico Mena Quintero <federico src gnome org>]

commit 20e8a3b3095ce6e26158c7e8f110a052d4c99262
Author: Federico Mena Quintero <federico gnome org>
Date:   Wed Sep 5 12:10:02 2018 -0500

    ViewParams: keep a Weak reference to the view box stack, not a strong one
    
    This lets us be extra-paranoid about not dropping a ViewParams after
    the corresponding DrawingCtx has been dropped.

 rsvg_internals/src/drawing_ctx.rs | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)
---
diff --git a/rsvg_internals/src/drawing_ctx.rs b/rsvg_internals/src/drawing_ctx.rs
index 60a7ad92..3c45f488 100644
--- a/rsvg_internals/src/drawing_ctx.rs
+++ b/rsvg_internals/src/drawing_ctx.rs
@@ -9,7 +9,7 @@ use pango_cairo_sys;
 use pango_sys;
 use pangocairo;
 use std::cell::RefCell;
-use std::rc::Rc;
+use std::rc::{Rc, Weak};
 
 use bbox::BoundingBox;
 use clip_path::{ClipPathUnits, NodeClipPath};
@@ -53,7 +53,7 @@ pub struct ViewParams {
     dpi_y: f64,
     view_box_width: f64,
     view_box_height: f64,
-    view_box_stack: Option<Rc<RefCell<Vec<ViewBox>>>>,
+    view_box_stack: Option<Weak<RefCell<Vec<ViewBox>>>>,
 }
 
 impl ViewParams {
@@ -87,7 +87,10 @@ impl ViewParams {
 
 impl Drop for ViewParams {
     fn drop(&mut self) {
-        if let Some(ref stack) = self.view_box_stack {
+        if let Some(ref weak_stack) = self.view_box_stack {
+            let stack = weak_stack
+                .upgrade()
+                .expect("A ViewParams was dropped after its DrawingCtx!?");
             stack.borrow_mut().pop();
         }
     }
@@ -258,7 +261,7 @@ impl<'a> DrawingCtx<'a> {
             dpi_y: self.dpi_y,
             view_box_width: width,
             view_box_height: height,
-            view_box_stack: Some(self.view_box_stack.clone()),
+            view_box_stack: Some(Rc::downgrade(&self.view_box_stack)),
         }
     }