Fix type_map() (#488)

We cannot declare `VipsTypeMap2Fn` as opaque as this callback is used
by `type_map()`. Instead, restore the `VipsTypeMap2Fn` definition and
use an opaque pointer for the second argument of `vips_type_map()`.

pyvips/vdecls.py

@@ -72,8 +72,8 @@ def cdefs(features):
         const char* g_type_name (GType gtype);
         GType g_type_from_name (const char* name);
 
-        typedef ... VipsTypeMap2Fn;
-        void* vips_type_map (GType base, VipsTypeMap2Fn fn, void* a, void* b);
+        typedef void* (*VipsTypeMap2Fn) (GType type, void* a, void* b);
+        void* vips_type_map (GType base, void* fn, void* a, void* b);
 
         const char* vips_error_buffer (void);
         void vips_error_clear (void);