Use convenient alists to manage per-frame font driver-specific data.

Use convenient alists to manage per-frame font driver-specific data.
* frame.h (struct frame): Rename font_data_list to...
[HAVE_XFT || HAVE_FREETYPE]: ... font_data, which is a Lisp_Object now.
* font.h (struct font_data_list): Remove; no longer need a special
data type.
(font_put_frame_data, font_get_frame_data) [HAVE_XFT || HAVE_FREETYPE]:
Adjust prototypes.
* font.c (font_put_frame_data, font_get_frame_data)
[HAVE_XFT || HAVE_FREETYPE]: Prefer alist functions to ad-hoc list
management.
* xftfont.c (xftfont_get_xft_draw, xftfont_end_for_frame):
Related users changed.
* ftxfont.c (ftxfont_get_gcs, ftxfont_end_for_frame): Likewise.
Prefer convenient xmalloc and xfree.