Geometry Nodes: add field support for socket inspection

Since fields were committed to master, socket inspection did
not work correctly for all socket types anymore. Now the same
functionality as before is back. Furthermore, fields that depend
on some input will now show the inputs in the socket inspection.

I added support for evaluating constant fields more immediately.
This has the benefit that the same constant field is not evaluated
more than once. It also helps with making the field independent
of the multi-functions that it uses. We might still want to change
the ownership handling for the multi-functions of nodes a bit,
but that can be done separately.

Differential Revision: https://developer.blender.org/D12444

source/blender/functions/FN_multi_function_builder.hh

@@ -326,18 +326,21 @@ template<typename From, typename To> class CustomMF_Convert : public MultiFuncti
 
 /**
  * A multi-function that outputs the same value every time. The value is not owned by an instance
- * of this function. The caller is responsible for destructing and freeing the value.
+ * of this function. If #make_value_copy is false, the caller is responsible for destructing and
+ * freeing the value.
  */
 class CustomMF_GenericConstant : public MultiFunction {
  private:
   const CPPType &type_;
   const void *value_;
   MFSignature signature_;
+  bool owns_value_;
 
   template<typename T> friend class CustomMF_Constant;
 
  public:
-  CustomMF_GenericConstant(const CPPType &type, const void *value);
+  CustomMF_GenericConstant(const CPPType &type, const void *value, bool make_value_copy);
+  ~CustomMF_GenericConstant();
   void call(IndexMask mask, MFParams params, MFContext context) const override;
   uint64_t hash() const override;
   bool equals(const MultiFunction &other) const override;