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.
(cherry picked from commit
536f48e9a2251b9e654ea974bd90ff2f40218753)