support exif res unit "none"

well why would you want that, but it seems some images use it

ChangeLog

@@ -1,6 +1,7 @@
 8/9/14 started 7.40.8
 - fix configure on rhel6 [Lovell]
 - mono <-> rgb converters were not handling extra bands, thanks James
+- support jpeg resunit "none"
 
 21/8/14 started 7.40.7
 - width and height were swapped in matlab load

libvips/foreign/jpeg2vips.c

@@ -57,6 +57,8 @@
  * 	- null-terminate exif strings, thanks Mike
  * 24/2/14
  * 	- don't write to our input buffer, thanks Lovell
+ * 9/9/14
+ * 	- support "none" as a resolution unit
  */
 
 /*
@@ -547,6 +549,13 @@ res_from_exif( VipsImage *im, ExifData *ed )
 #endif /*DEBUG*/
 
 	switch( unit ) {
+	case 1:
+		/* No unit ... just pass the fields straight to vips.
+		 */
+		vips_image_set_string( im, 
+			VIPS_META_RESOLUTION_UNIT, "none" );
+		break;
+
 	case 2:
 		/* In inches.
 		 */

libvips/foreign/vips2jpeg.c

@@ -63,6 +63,8 @@
  * 	- add "strip" option to remove all metadata
  * 13/11/13
  * 	- add a "no_subsample" option to disable chroma subsample
+ * 9/9/14
+ * 	- support "none" as a resolution unit
  */
 
 /*
@@ -460,11 +462,19 @@ set_exif_resolution( ExifData *ed, VipsImage *im )
 	 */
 	unit = 2;
 	if( vips_image_get_typeof( im, VIPS_META_RESOLUTION_UNIT ) &&
-		!vips_image_get_string( im, VIPS_META_RESOLUTION_UNIT, &p ) &&
-		vips_isprefix( "cm", p ) ) 
-		unit = 3;
+		!vips_image_get_string( im, VIPS_META_RESOLUTION_UNIT, &p ) ) {
+		if( vips_isprefix( "cm", p ) ) 
+			unit = 3;
+		else if( vips_isprefix( "none", p ) ) 
+			unit = 1;
+	}
 
 	switch( unit ) {
+	case 1:
+		xres = im->Xres;
+		yres = im->Yres;
+		break;
+
 	case 2:
 		xres = im->Xres * 25.4;
 		yres = im->Yres * 25.4;