support rendering of wider range of combinging characters by ftfont backend

* lisp/language/hebrew.el (hebrew-shape-gstring): If the font backend
supports rendering of combining characters, call
font-shape-gstring.

* src/font.c (Ffont_get): Handle `combining-capability' property.
(syms_of_font): New symbol ":combining-capability'.

* src/font.h (struct font_driver): New member combining_capability.

* src/ftfont.c: Include "category.h".
(ftfont_driver): Initialize combining_capability to
ftfont_combining_capability.
(ftfont_shape_by_flt): If OTF is null, try to find a suitable
FLT in advance.
(ftfont_combining_capability): New function.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/font.h b/src/font.h
index 1d13e1cb6b0..f6f862ac7df 100644
--- a/src/font.h
+++ b/src/font.h
@@ -715,6 +715,12 @@ struct font_driver
   bool (*cached_font_ok) (struct frame *f,
                           Lisp_Object font_object,
                           Lisp_Object entity);
+
+  /* Optional
+
+     Return non-nil if the driver support rendering of combining
+     characters for FONT according to Unicode combining class.  */
+  Lisp_Object (*combining_capability) (struct font *font);
 };