vips_image_write() only refs input when it has to

when you write to a non-partial image, you create a sink ... so
vips_image_write() only needs to ref the input when writing to partials

this change makes it much easier to (for example) combine many images in
bounded space, see for example:

https://gist.github.com/jcupitt/c516325ebef7601b5da610af5619c9b2

ChangeLog

@@ -17,6 +17,8 @@
 - free pixel buffers on image close as well as thread exit ... stops main 
   thread buffers clogging up the system
 - 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
 
 18/5/16 started 8.3.2
 - more robust vips image reading

libvips/iofuncs/image.c

@@ -8,6 +8,8 @@
  * 	- add vips_image_copy_memory()
  * 25/11/15
  * 	- add vips_image_new_from_memory_copy()
+ * 10/6/16
+ * 	- vips_image_write() does not ref input for non-partial images
  */
 
 /*
@@ -2448,17 +2450,22 @@ vips_image_write( VipsImage *image, VipsImage *out )
 			VIPS_DEMAND_STYLE_THINSTRIP, image, NULL ) )
 		return( -1 );
 
-	/* We generate from @image partially, so we need to keep it about as
-	 * long as @out is about. 
-	 */
-	g_object_ref( image );
-	vips_object_local( out, image );
-
 	if( vips_image_generate( out,
 		vips_start_one, vips_image_write_gen, vips_stop_one, 
 		image, NULL ) )
 		return( -1 );
 
+	/* If @out is a partial image, we need to make sure that @image stays
+	 * alive as long as @out is alive.
+	 *
+	 * If it's not partial, perhaps a file we write to, or a memory image,
+	 * it's fine for @image to go away.
+	 */
+	if( vips_image_ispartial( out ) ) { 
+		g_object_ref( image );
+		vips_object_local( out, image );
+	}
+
 	return( 0 );
 }