[librsvg/attribute-parsers-737: 19/26] FilterContext: extract method to find a primitive's default input

[Federico Mena Quintero <federico src gnome org>]

commit 7b39b4ae2e2905c4ff51c63512079ee7c4f02c05
Author: Federico Mena Quintero <federico gnome org>
Date:   Thu Oct 20 18:19:24 2022 -0500

    FilterContext: extract method to find a primitive's default input

 src/filters/context.rs | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)
---
diff --git a/src/filters/context.rs b/src/filters/context.rs
index 47ae32afb..17a746f80 100644
--- a/src/filters/context.rs
+++ b/src/filters/context.rs
@@ -280,6 +280,20 @@ impl FilterContext {
         self.effects_region
     }
 
+    /// Get a filter primitive's default input as if its `in=\"...\"` were not specified.
+    ///
+    /// Per https://drafts.fxtf.org/filter-effects/#element-attrdef-filter-primitive-in,
+    /// "References to non-existent results will be treated as if no result was
+    /// specified".  That is, fall back to the last result in the filter chain, or if this
+    /// is the first in the chain, just use SourceGraphic.
+    fn get_unspecified_input(&self) -> FilterInput {
+        if let Some(output) = self.last_result.as_ref() {
+            FilterInput::PrimitiveOutput(output.clone())
+        } else {
+            FilterInput::StandardInput(self.source_graphic().clone())
+        }
+    }
+
     /// Retrieves the filter input surface according to the SVG rules.
     fn get_input_raw(
         &self,
@@ -288,16 +302,7 @@ impl FilterContext {
         in_: &Input,
     ) -> Result<FilterInput, FilterError> {
         match *in_ {
-            Input::Unspecified => {
-                // No value => use the last result.
-                // As per the SVG spec, if the filter primitive is the first in the chain, return the
-                // source graphic.
-                if let Some(output) = self.last_result.as_ref() {
-                    Ok(FilterInput::PrimitiveOutput(output.clone()))
-                } else {
-                    Ok(FilterInput::StandardInput(self.source_graphic().clone()))
-                }
-            }
+            Input::Unspecified => Ok(self.get_unspecified_input()),
 
             Input::SourceGraphic => Ok(FilterInput::StandardInput(self.source_graphic().clone())),