don't cache write operations

we want writes to happen every time, in case the file has been deleted
by someone

ChangeLog

@@ -4,6 +4,7 @@
 - png save compression range was wrong
 - more/moreeq was wrong
 - vips7 ppm save with options was broken 
+- don't cache write operations
 
 18/6/12 started 7.28.9
 - slightly more memory debugging output

libvips/foreign/foreign.c

@@ -1375,6 +1375,10 @@ vips_foreign_save_build( VipsObject *object )
 		vips_foreign_convert_saveable( save ) )
 		return( -1 );
 
+	/* You mustn't cache write operations.
+	 */
+	vips_operation_set_nocache( VIPS_OPERATION( save ), TRUE );
+
 	if( VIPS_OBJECT_CLASS( vips_foreign_save_parent_class )->
 		build( object ) )
 		return( -1 );

libvips/foreign/vips2tiff.c

@@ -464,7 +464,7 @@ write_tiff_header( TiffWrite *tw, TIFF *tif, int width, int height )
 		TIFFSetField( tif, TIFFTAG_JPEGCOLORMODE, JPEGCOLORMODE_RGB );
 	}
 
-	if( tw->predictor != -1 ) 
+	if( tw->predictor != VIPS_FOREIGN_TIFF_PREDICTOR_NONE ) 
 		TIFFSetField( tif, TIFFTAG_PREDICTOR, tw->predictor );
 
 	/* Don't write mad resolutions (eg. zero), it confuses some programs.
@@ -1338,7 +1338,7 @@ tiff_copy( TiffWrite *tw, TIFF *out, TIFF *in )
 	CopyField( TIFFTAG_ROWSPERSTRIP, i32 );
 	CopyField( TIFFTAG_SUBFILETYPE, i32 );
 
-	if( tw->predictor != -1 ) 
+	if( tw->predictor != VIPS_FOREIGN_TIFF_PREDICTOR_NONE ) 
 		TIFFSetField( out, TIFFTAG_PREDICTOR, tw->predictor );
 
 	/* TIFFTAG_JPEGQUALITY is a pesudo-tag, so we can't copy it.