fix mono fits write

we should write mono images as naxis=2

see https://github.com/libvips/libvips/issues/3265

ChangeLog

@@ -4,6 +4,7 @@ xx/1/23 8.14.1
 - add vips_semaphore_down_timeout() [kleisauke]
 - idle threads are removed after 15s [kleisauke]
 - fix version number in gtk-doc index [kleisauke]
+- save mono fits images as NAXIS=2 [ewelot]
 
 22/12/22 8.14.0
 

libvips/foreign/fits.c

@@ -36,6 +36,8 @@
  * 27/10/22
  *      - band interleave ourselves on read
  *      - don't duplicate metadata
+ * 6/1/23 ewelot
+ *	- save mono images as NAXIS=2
  */
 
 /*
@@ -658,7 +660,7 @@ vips_fits_set_header( VipsFits *fits, VipsImage *in )
 
 	status = 0;
 
-	fits->naxis = 3;
+	fits->naxis = in->Bands == 1 ? 2 : 3;
 	fits->naxes[0] = in->Xsize;
 	fits->naxes[1] = in->Ysize;
 	fits->naxes[2] = in->Bands;