set magick-format

see https://github.com/libvips/libvips/discussions/3011

libvips/foreign/magick2vips.c

@@ -66,6 +66,8 @@
  * 	- add profile (xmp, ipct, etc.) read
  * 12/11/21
  * 	- set "orientation"
+ * 26/8/22
+ *      - set "magick-format"
  */
 
 /*
@@ -485,6 +487,13 @@ parse_header( Read *read )
 }
 #endif 
 
+        /* Something like "BMP".
+         */
+        if( read->image->magick &&
+                strlen( read->image->magick ) > 0 )
+                vips_image_set_string( im, "magick-format", 
+                        read->image->magick );
+
 	/* Do we have a set of equal-sized frames? Append them.
 
 	   	FIXME ... there must be an attribute somewhere from dicom read