[librsvg] drawing_ctx: rework get_snapshot

[Paolo Borelli <pborelli src gnome org>]

commit b0959e07bdd204392a309532da1e00212be96342
Author: Paolo Borelli <pborelli gnome org>
Date:   Fri Jan 10 22:27:25 2020 +0100

    drawing_ctx: rework get_snapshot
    
    Return a SharedImageSurface, this makes the calling code cleaner
    and independent from cairo.

 rsvg_internals/src/drawing_ctx.rs     | 35 +++++++++++++++++++++++------------
 rsvg_internals/src/filters/context.rs | 13 +++----------
 2 files changed, 26 insertions(+), 22 deletions(-)
---
diff --git a/rsvg_internals/src/drawing_ctx.rs b/rsvg_internals/src/drawing_ctx.rs
index 4fe72161..8409f3f2 100644
--- a/rsvg_internals/src/drawing_ctx.rs
+++ b/rsvg_internals/src/drawing_ctx.rs
@@ -899,7 +899,11 @@ impl DrawingCtx {
         cr.clip();
     }
 
-    pub fn get_snapshot(&self, surface: &cairo::ImageSurface) {
+    pub fn get_snapshot(
+        &self,
+        width: i32,
+        height: i32,
+    ) -> Result<SharedImageSurface, cairo::Status> {
         // TODO: as far as I can tell this should not render elements past the last (topmost) one
         // with enable-background: new (because technically we shouldn't have been caching them).
         // Right now there are no enable-background checks whatsoever.
@@ -915,18 +919,25 @@ impl DrawingCtx {
         //
         // CSS Compositing and Blending, "isolation" property:
         //   https://www.w3.org/TR/compositing-1/#isolation
-        let cr = cairo::Context::new(&surface);
-        for (depth, draw) in self.cr_stack.iter().enumerate() {
-            let affines = CompositingAffines::new(
-                draw.get_matrix(),
-                self.initial_affine_with_offset(),
-                depth,
-            );
-
-            cr.set_matrix(affines.for_snapshot);
-            cr.set_source_surface(&draw.get_target(), 0.0, 0.0);
-            cr.paint();
+        let surface = cairo::ImageSurface::create(cairo::Format::ARgb32, width, height)?;
+
+        {
+            let cr = cairo::Context::new(&surface);
+
+            for (depth, draw) in self.cr_stack.iter().enumerate() {
+                let affines = CompositingAffines::new(
+                    draw.get_matrix(),
+                    self.initial_affine_with_offset(),
+                    depth,
+                );
+
+                cr.set_matrix(affines.for_snapshot);
+                cr.set_source_surface(&draw.get_target(), 0.0, 0.0);
+                cr.paint();
+            }
         }
+
+        SharedImageSurface::wrap(surface, SurfaceType::SRgb)
     }
 
     pub fn lookup_image(&self, href: &str) -> Result<SharedImageSurface, RenderingError> {
diff --git a/rsvg_internals/src/filters/context.rs b/rsvg_internals/src/filters/context.rs
index 7f923786..4c242c8f 100644
--- a/rsvg_internals/src/filters/context.rs
+++ b/rsvg_internals/src/filters/context.rs
@@ -198,16 +198,9 @@ impl FilterContext {
         let mut bg = self.background_surface.borrow_mut();
 
         *bg = Some(
-            cairo::ImageSurface::create(
-                cairo::Format::ARgb32,
-                self.source_surface.width(),
-                self.source_surface.height(),
-            )
-            .map_err(FilterError::CairoError)
-            .and_then(|s| {
-                draw_ctx.get_snapshot(&s);
-                SharedImageSurface::wrap(s, SurfaceType::SRgb).map_err(FilterError::CairoError)
-            }),
+            draw_ctx
+                .get_snapshot(self.source_surface.width(), self.source_surface.height())
+                .map_err(FilterError::CairoError),
         );
 
         // Return the only existing reference as immutable.