Document font structure layout constraints

The layout of the initial members of ftcrfont_info must match
ftfont_info

* src/ftcrfont.c (struct ftcrfont_info): Likewise.

* src/ftfont.c (struct ftfont_info): Document layout constraints.

src/ftcrfont.c

@@ -35,8 +35,9 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 struct ftcrfont_info
 {
   struct font font;
-  /* The following six members must be here in this order to be
-     compatible with struct ftfont_info (in ftfont.c).  */
+  /* The following members up to and including 'matrix' must be here
+     in this order to be compatible with struct ftfont_info (in
+     ftfont.c).  */
 #ifdef HAVE_LIBOTF
   bool maybe_otf;	  /* Flag to tell if this may be OTF or not.  */
   OTF *otf;

src/ftfont.c

@@ -56,8 +56,9 @@ struct ftfont_info
 {
   struct font font;
 #ifdef HAVE_LIBOTF
-  /* The following four members must be here in this order to be
-     compatible with struct xftfont_info (in xftfont.c).  */
+  /* The following members up to and including 'matrix' must be here in
+     this order to be compatible with struct xftfont_info (in
+     xftfont.c).  */
   bool maybe_otf;	/* Flag to tell if this may be OTF or not.  */
   OTF *otf;
 #endif	/* HAVE_LIBOTF */