[librsvg] offset: move cairo implementation to surface_utils

[Paolo Borelli <pborelli src gnome org>]

commit 444f32240db9db72b5ac391572301f2ff485f495
Author: Paolo Borelli <pborelli gnome org>
Date:   Mon Jan 6 12:40:40 2020 +0100

    offset: move cairo implementation to surface_utils

 rsvg_internals/src/filters/offset.rs               | 29 +++-------------------
 rsvg_internals/src/surface_utils/shared_surface.rs | 29 ++++++++++++++++++++++
 2 files changed, 32 insertions(+), 26 deletions(-)
---
diff --git a/rsvg_internals/src/filters/offset.rs b/rsvg_internals/src/filters/offset.rs
index 69647cf5..b7c3389b 100644
--- a/rsvg_internals/src/filters/offset.rs
+++ b/rsvg_internals/src/filters/offset.rs
@@ -4,7 +4,6 @@ use crate::drawing_ctx::DrawingCtx;
 use crate::node::{NodeResult, NodeTrait, RsvgNode};
 use crate::parsers::ParseValue;
 use crate::property_bag::PropertyBag;
-use crate::surface_utils::shared_surface::SharedImageSurface;
 
 use super::context::{FilterContext, FilterOutput, FilterResult};
 use super::{FilterEffect, FilterError, PrimitiveWithInput};
@@ -60,35 +59,13 @@ impl FilterEffect for FeOffset {
             .add_input(&input)
             .into_irect(draw_ctx);
 
-        let (ox, oy) = ctx.paffine().transform_distance(self.dx, self.dy);
+        let (dx, dy) = ctx.paffine().transform_distance(self.dx, self.dy);
 
-        let output_surface = cairo::ImageSurface::create(
-            cairo::Format::ARgb32,
-            ctx.source_graphic().width(),
-            ctx.source_graphic().height(),
-        )?;
-
-        // output_bounds contains all pixels within bounds,
-        // for which (x - ox) and (y - oy) also lie within bounds.
-        if let Some(output_bounds) = bounds
-            .translate((ox as i32, oy as i32))
-            .intersection(&bounds)
-        {
-            let cr = cairo::Context::new(&output_surface);
-            let r = cairo::Rectangle::from(output_bounds);
-            cr.rectangle(r.x, r.y, r.width, r.height);
-            cr.clip();
-
-            input.surface().set_as_source_surface(&cr, ox, oy);
-            cr.paint();
-        }
+        let surface = input.surface().offset(bounds, dx, dy)?;
 
         Ok(FilterResult {
             name: self.base.result.clone(),
-            output: FilterOutput {
-                surface: SharedImageSurface::new(output_surface, input.surface().surface_type())?,
-                bounds,
-            },
+            output: FilterOutput { surface, bounds },
         })
     }
 
diff --git a/rsvg_internals/src/surface_utils/shared_surface.rs 
b/rsvg_internals/src/surface_utils/shared_surface.rs
index 5dd53979..4454109d 100644
--- a/rsvg_internals/src/surface_utils/shared_surface.rs
+++ b/rsvg_internals/src/surface_utils/shared_surface.rs
@@ -932,6 +932,35 @@ impl SharedImageSurface {
         SharedImageSurface::new(output_surface, self.surface_type)
     }
 
+    /// Offsets the image of the specified amount.
+    #[inline]
+    pub fn offset(
+        &self,
+        bounds: IRect,
+        dx: f64,
+        dy: f64
+    ) -> Result<SharedImageSurface, cairo::Status> {
+        let output_surface =
+            cairo::ImageSurface::create(cairo::Format::ARgb32, self.width, self.height)?;
+
+        // output_bounds contains all pixels within bounds,
+        // for which (x - ox) and (y - oy) also lie within bounds.
+        if let Some(output_bounds) = bounds
+            .translate((dx as i32, dy as i32))
+            .intersection(&bounds)
+        {
+            let cr = cairo::Context::new(&output_surface);
+            let r = cairo::Rectangle::from(output_bounds);
+            cr.rectangle(r.x, r.y, r.width, r.height);
+            cr.clip();
+
+            self.set_as_source_surface(&cr, dx, dy);
+            cr.paint();
+        }
+
+        SharedImageSurface::new(output_surface, self.surface_type)
+    }
+
     /// Performs the combination of two input surfaces using Porter-Duff
     /// compositing operators
     ///