[librsvg] filters: shorten "merge" code

[Paolo Borelli <pborelli src gnome org>]

commit f68fbc05a5cae66a6c777444d2213e1aaeeb590c
Author: Paolo Borelli <pborelli gnome org>
Date:   Tue Jan 14 01:45:28 2020 +0100

    filters: shorten "merge" code

 rsvg_internals/src/filters/merge.rs | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)
---
diff --git a/rsvg_internals/src/filters/merge.rs b/rsvg_internals/src/filters/merge.rs
index f650fa59..b8b95cde 100644
--- a/rsvg_internals/src/filters/merge.rs
+++ b/rsvg_internals/src/filters/merge.rs
@@ -115,24 +115,18 @@ impl FilterEffect for FeMerge {
             )?);
         }
 
-        let output_surface = match output_surface {
-            Some(surface) => surface,
-            None => SharedImageSurface::wrap(
-                cairo::ImageSurface::create(
-                    cairo::Format::ARgb32,
-                    ctx.source_graphic().width(),
-                    ctx.source_graphic().height(),
-                )?,
+        let surface = match output_surface {
+            Some(s) => s,
+            None => SharedImageSurface::empty(
+                ctx.source_graphic().width(),
+                ctx.source_graphic().height(),
                 SurfaceType::AlphaOnly,
             )?,
         };
 
         Ok(FilterResult {
             name: self.base.result.clone(),
-            output: FilterOutput {
-                surface: output_surface,
-                bounds,
-            },
+            output: FilterOutput { surface, bounds },
         })
     }