more progressive jpg changes

- Remove the nocache stuff for prog jpg images. There was no saving in
  practice, since operations downstream could be cached.

  nocache is for objects that can change, not for memory saving

- Call _destroy on the decompress object much earlier, it frees a huge
  amount of memoey for prog jpg images.

ChangeLog

@@ -6,7 +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
+- lower mem use for progressive jpg decode
 
 27/1/16 started 8.2.3
 - fix a crash with SPARC byte-order labq vips images

libvips/foreign/jpeg2vips.c

@@ -69,6 +69,9 @@
  * 	  fd during jpg read, handy for large numbers of input images 
  * 15/7/15
  * 	- save exif tags using @name, not @title ... @title is subject to i18n
+ * 21/2/16
+ * 	- _destroy the decompress object as soon as we can, frees loads of
+ * 	  memory for progressive jpg files
  */
 
 /*
@@ -977,6 +980,12 @@ read_jpeg_generate( VipsRegion *or,
 		jpeg->y_pos += 1; 
 	}
 
+	/* Progressive images can have a lot of memory in the decompress
+	 * object, destroy as soon as we can. Safe to call many times. 
+	 */
+	if( jpeg->y_pos >= or->im->Ysize ) 
+		jpeg_destroy_decompress( &jpeg->cinfo );
+
 	VIPS_GATE_STOP( "read_jpeg_generate: work" );
 
 	return( 0 );
@@ -1083,7 +1092,6 @@ vips__jpeg_read( ReadJpeg *jpeg, VipsImage *out, gboolean header_only )
 }
 #endif /*DEBUG*/
 
-
 	/* Convert!
 	 */
 	if( header_only ) {

libvips/foreign/jpegload.c

@@ -4,8 +4,6 @@
  * 	- wrap a class around the jpeg writer
  * 29/11/11
  * 	- split to make load, load from buffer and load from file
- * 18/2/16
- * 	- don't cache progressive jpgs, they take too much mem
  */
 
 /*
@@ -94,25 +92,6 @@ typedef VipsForeignLoadClass VipsForeignLoadJpegClass;
 G_DEFINE_ABSTRACT_TYPE( VipsForeignLoadJpeg, vips_foreign_load_jpeg, 
 	VIPS_TYPE_FOREIGN_LOAD );
 
-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_get_flags( VipsForeignLoad *load )
 {
@@ -147,7 +126,6 @@ vips_foreign_load_jpeg_class_init( VipsForeignLoadJpegClass *class )
 {
 	GObjectClass *gobject_class = G_OBJECT_CLASS( class );
 	VipsObjectClass *object_class = (VipsObjectClass *) class;
-	VipsOperationClass *operation_class = (VipsOperationClass *) class;
 	VipsForeignLoadClass *load_class = (VipsForeignLoadClass *) class;
 
 	gobject_class->set_property = vips_object_set_property;
@@ -157,8 +135,6 @@ vips_foreign_load_jpeg_class_init( VipsForeignLoadJpegClass *class )
 	object_class->description = _( "load jpeg" );
 	object_class->build = vips_foreign_load_jpeg_build;
 
-	operation_class->get_flags = vips_foreign_load_jpeg_operation_get_flags;
-
 	load_class->get_flags = vips_foreign_load_jpeg_get_flags;
 
 	VIPS_ARG_INT( class, "shrink", 10,