don't cache progressive jpg images

progressive jpg images need loads of memory, we shouldn't cache them by
default ... our called can hold a ref if they want to keep the image
around

see https://github.com/jcupitt/libvips/issues/387

ChangeLog

@@ -6,6 +6,7 @@
 - vipsthumbnail knows about pdfload and svgload
 - added @page param to magickload 
 - matload is more specific (thanks bithive)
+- don't cache progressive jpg images
 
 27/1/16 started 8.2.3
 - fix a crash with SPARC byte-order labq vips images

libvips/foreign/foreign.c

@@ -928,6 +928,7 @@ static VipsOperationFlags
 vips_foreign_load_operation_get_flags( VipsOperation *operation )
 {
 	VipsForeignLoad *load = VIPS_FOREIGN_LOAD( operation );
+
 	VipsOperationFlags flags;
 
 	flags = VIPS_OPERATION_CLASS( vips_foreign_load_parent_class )->

libvips/foreign/jpegload.c

@@ -179,6 +179,25 @@ typedef VipsForeignLoadJpegClass VipsForeignLoadJpegFileClass;
 G_DEFINE_TYPE( VipsForeignLoadJpegFile, vips_foreign_load_jpeg_file, 
 	vips_foreign_load_jpeg_get_type() );
 
+static VipsOperationFlags 
+vips_foreign_load_jpeg_operation_get_flags( VipsOperation *operation )
+{
+	VipsForeignLoad *load = VIPS_FOREIGN_LOAD( operation );
+
+	VipsOperationFlags flags;
+	int multiscan;
+
+	flags = VIPS_OPERATION_CLASS( vips_foreign_load_jpeg_parent_class )->
+		get_flags( operation );
+	if( load->out &&
+		!vips_image_get_int( load->out, 
+			"jpeg-multiscan", &multiscan ) &&
+		multiscan ) 
+		flags |= VIPS_OPERATION_NOCACHE;
+
+	return( flags );
+}
+
 static VipsForeignFlags
 vips_foreign_load_jpeg_file_get_flags_filename( const char *filename )
 {
@@ -229,6 +248,7 @@ vips_foreign_load_jpeg_file_class_init( VipsForeignLoadJpegFileClass *class )
 {
 	GObjectClass *gobject_class = G_OBJECT_CLASS( class );
 	VipsObjectClass *object_class = (VipsObjectClass *) class;
+	VipsOperationClass *operation_class = (VipsOperationClass *) class;
 	VipsForeignClass *foreign_class = (VipsForeignClass *) class;
 	VipsForeignLoadClass *load_class = (VipsForeignLoadClass *) class;
 
@@ -238,6 +258,8 @@ vips_foreign_load_jpeg_file_class_init( VipsForeignLoadJpegFileClass *class )
 	object_class->nickname = "jpegload";
 	object_class->description = _( "load jpeg from file" );
 
+	operation_class->get_flags = vips_foreign_load_jpeg_operation_get_flags;
+
 	foreign_class->suffs = jpeg_suffs;
 
 	/* We are fast at is_a(), so high priority.