add implementation for VImage::write()

oops, it was missing

also, change the return type from void to VImage. This makes chaining
possible, eg.:

   VImage memory = im.write( Viameg::new_memory() );

ChangeLog

@@ -19,6 +19,8 @@
 - dzsave can write compressed zips [Felix Bünemann]
 - vips_image_write() only refs the input when it has to ... makes it easier to
   combine many images in bounded memory
+- VImage::write() implementation was missing
+- VImage::write() return value changed from void to VImage to help chaining
 
 18/5/16 started 8.3.2
 - more robust vips image reading

cplusplus/VImage.cpp

@@ -2,6 +2,8 @@
  *
  * 30/12/14
  * 	- allow set enum value from string
+ * 10/6/16
+ * 	- missing implementation of VImage::write()
  */
 
 /*
@@ -612,6 +614,15 @@ VImage::new_matrixv( int width, int height, ... )
 	return( matrix ); 
 }
 
+VImage
+VImage::write( VImage out )
+{
+	if( vips_image_write( this->get_image(), out.get_image() ) )
+		throw VError(); 
+
+	return( out ); 
+}
+
 void 
 VImage::write_to_file( const char *name, VOption *options )
 {

cplusplus/include/vips/VImage8.h

@@ -471,7 +471,7 @@ public:
 	VImage new_from_image( std::vector<double> pixel );
 	VImage new_from_image( double pixel );
 
-	void write( VImage out );
+	VImage write( VImage out );
 
 	void write_to_file( const char *name, VOption *options = 0 );