revert an accidental ABI break

the new window manager code in 8.6.1 changed the prototype of an internal
function which was visible in the public API

this patch renames the function, adds it to an internal header instead,
and adds a stub for the old prototype

see https://github.com/jcupitt/libvips/issues/854

ChangeLog

@@ -1,5 +1,6 @@
 5/1/18 started 8.6.2
 - vips_sink_screen() keeps a ref to the input image ... stops a rare race
+- fix a minor accidental ABI break in 8.6.0 -> 8.6.1 [remicollet]
 
 10/12/17 started 8.6.1
 - fix mmap window new/free cycling 

libvips/include/vips/internal.h

@@ -257,6 +257,11 @@ void vips__reorder_init( void );
 int vips__reorder_set_input( VipsImage *image, VipsImage **in );
 void vips__reorder_clear( VipsImage *image );
 
+/* Window manager API.
+ */
+VipsWindow *vips_window_take( VipsWindow *window, 
+	VipsImage *im, int top, int height );
+
 #ifdef __cplusplus
 }
 #endif /*__cplusplus*/

libvips/include/vips/private.h

@@ -71,10 +71,6 @@ typedef struct {
 	size_t length;		/* Size of window */
 } VipsWindow;
 
-/* window manager.
- */
-VipsWindow *vips_window_ref( VipsWindow *window, 
-	struct _VipsImage *im, int top, int height );
 int vips_window_unref( VipsWindow *window );
 void vips_window_print( VipsWindow *window );
 

libvips/include/vips/vips7compat.h

@@ -1227,6 +1227,10 @@ int vips_check_dmask_1d( const char *domain, DOUBLEMASK *mask );
 
 GOptionGroup *vips_get_option_group( void );
 
+/* old window manager API
+ */
+VipsWindow *vips_window_ref( VipsImage *im, int top, int height );
+
 #ifdef __cplusplus
 }
 #endif /*__cplusplus*/

libvips/iofuncs/region.c

@@ -678,7 +678,7 @@ vips_region_image( VipsRegion *reg, const VipsRect *r )
 		/* No complete image data ... but we can use a rolling window.
 		 */
 		reg->type = VIPS_REGION_WINDOW;
-		if( !(reg->window = vips_window_ref( reg->window, image, 
+		if( !(reg->window = vips_window_take( reg->window, image, 
 			clipped.top, clipped.height )) )
 			return( -1 );
 

libvips/iofuncs/window.c

@@ -342,10 +342,18 @@ vips_window_find( VipsImage *im, int top, int height )
 	return( window );
 }
 
-/* Update a ref to a window to make it enclose top/height.
+/* Old API. Just a compat stub now.
  */
 VipsWindow *
-vips_window_ref( VipsWindow *window, VipsImage *im, int top, int height )
+vips_window_ref( VipsImage *im, int top, int height )
+{
+	return( NULL );
+}
+
+/* Update a window to make it enclose top/height. 
+ */
+VipsWindow *
+vips_window_take( VipsWindow *window, VipsImage *im, int top, int height )
 {
 	int margin;