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, 20 insertions, 18 deletions

diff --git a/lisp/language/hebrew.el b/lisp/language/hebrew.el
index a3f4b3dd997..05f2f2521f7 100644
--- a/lisp/language/hebrew.el
+++ b/lisp/language/hebrew.el
@@ -216,24 +216,26 @@ Bidirectional editing is supported.")))
           (setq idx 1 nglyphs nchars))
         ;; Now IDX is an index to the first non-precomposed glyph.
         ;; Adjust positions of the remaining glyphs artificially.
-        (setq base-width (lglyph-width (lgstring-glyph gstring 0)))
-        (while (< idx nglyphs)
-          (setq glyph (lgstring-glyph gstring idx))
-          (lglyph-set-from-to glyph 0 (1- nchars))
-          (if (>= (lglyph-lbearing glyph) (lglyph-width glyph))
-              ;; It seems that this glyph is designed to be rendered
-              ;; before the base glyph.
-              (lglyph-set-adjustment glyph (- base-width) 0 0)
-            (if (>= (lglyph-lbearing glyph) 0)
-                ;; Align the horizontal center of this glyph to the
-                ;; horizontal center of the base glyph.
-                (let ((width (- (lglyph-rbearing glyph)
-                                (lglyph-lbearing glyph))))
-                  (lglyph-set-adjustment glyph
-                                         (- (/ (- base-width width) 2)
-                                            (lglyph-lbearing glyph)
-                                            base-width) 0 0))))
-          (setq idx (1+ idx))))))
+        (if (font-get font :combining-capability)
+            (font-shape-gstring gstring)
+          (setq base-width (lglyph-width (lgstring-glyph gstring 0)))
+          (while (< idx nglyphs)
+            (setq glyph (lgstring-glyph gstring idx))
+            (lglyph-set-from-to glyph 0 (1- nchars))
+            (if (>= (lglyph-lbearing glyph) (lglyph-width glyph))
+                ;; It seems that this glyph is designed to be rendered
+                ;; before the base glyph.
+                (lglyph-set-adjustment glyph (- base-width) 0 0)
+              (if (>= (lglyph-lbearing glyph) 0)
+                  ;; Align the horizontal center of this glyph to the
+                  ;; horizontal center of the base glyph.
+                  (let ((width (- (lglyph-rbearing glyph)
+                                  (lglyph-lbearing glyph))))
+                    (lglyph-set-adjustment glyph
+                                           (- (/ (- base-width width) 2)
+                                              (lglyph-lbearing glyph)
+                                              base-width) 0 0))))
+            (setq idx (1+ idx)))))))
     gstring))
 
 (let* ((base "[\u05D0-\u05F2]")