set "interlaced" for jpg and png

set "interlaced" for interlaced png and jpg images

ChangeLog

@@ -13,6 +13,7 @@
 - better hasalpha rules
 - create funcs always make MULTIBAND (ie. no alpha)
 - use O_TMPFILE, if available [Alexander--]
+- set "interlaced=1" for interlaced JPG and PNG images
 
 12/3/18 started 8.6.4
 - better fitting of fonts with overhanging edges, thanks Adrià 

libvips/foreign/jpeg2vips.c

@@ -94,6 +94,8 @@
  * 	- revert previous warning change: libvips reports serious corruption, 
  * 	  like a truncated file, as a warning and we need to be able to catch
  * 	  that
+ * 9/4/18
+ * 	- set interlaced=1 for interlaced images
  */
 
 /*
@@ -414,6 +416,11 @@ read_jpeg_header( ReadJpeg *jpeg, VipsImage *out )
 	(void) vips_image_set_int( out, "jpeg-multiscan", 
 		jpeg_has_multiple_scans( cinfo ) );
 
+	/* 8.7 adds this for PNG as well, so we have a new format-neutral name.
+	 */
+	if( jpeg_has_multiple_scans( cinfo ) )
+		vips_image_set_int( out, "interlaced", 1 ); 
+
 	/* Look for EXIF and ICC profile.
 	 */
 	for( p = cinfo->marker_list; p; p = p->next ) {

libvips/foreign/vipspng.c

@@ -65,6 +65,8 @@
  * 	- better behaviour for truncated png files, thanks Yury
  * 26/4/17
  * 	- better @fail handling with truncated PNGs
+ * 9/4/18
+ * 	- set interlaced=1 for interlaced images
  */
 
 /*
@@ -438,6 +440,11 @@ png2vips_header( Read *read, VipsImage *out )
 		return( -1 );
 	}
 
+	/* Let our caller know. These are very expensive to decode.
+	 */
+	if( interlace_type != PNG_INTERLACE_NONE ) 
+		vips_image_set_int( out, "interlaced", 1 ); 
+
 	return( 0 );
 }