[ft] Open blob in READONLY mode

HB_MEMORY_MODE_READONLY_MAY_MAKE_WRITABLE is deprecated and fairly
useless now.
Fix FreeType version check
Remove redundant check for FT_Face_GetCharVariantIndex

We require FreeType >= 2.8.3.  This symbol was introduced earlier
than that.
[ft] Add hb_ft_face_create_referenced() and hb_ft_font_create_referenced()

When I originally wrote hb-ft, FreeType objects did not support reference
counting.  As such, hb_ft_face_create() and hb_ft_font_create() had a
"destroy" callback and client was responsible for making sure FT_Face is
kept around as long as the hb-font/face are alive.

However, since this was not clearly documented, some clienets didn't
correctly did that.  In particular, some clients assumed that it's safe
to destroy FT_Face and then hb_face_t.  This, indeed, used to work, until
45fd9424c723f115ca98995b8f8a25185a6fc71d, which make face destroy access
font tables.

Now, I fixed that issue in 395b35903e052aecc97d0807e4f813c64c0d2b0b since
the access was not needed, but the problem remains that not all clients
handle this correctly.  See:

  https://bugs.freedesktop.org/show_bug.cgi?id=86300

Fortunately, FT_Reference_Face() was added to FreeType in 2010, and so we
can use it now.  Originally I wanted to change hb_ft_face_create() and
hb_ft_font_create() to reference the face if destroy==NULL was passed in.
That would improve pretty much all clients, with little undesired effects.
Except that FreeType itself, when compiled with HarfBuzz support, calls
hb_ft_font_create() with destroy==NULL and saves the resulting hb-font on
the ft-face (why does it not free it immediately?).  Making hb-face
reference ft-face causes a cycling reference there.  At least, that's my
current understanding.

At any rate, a cleaner approach, even if it means all clients will need a
change, is to introduce brand new API.  Which this commit does.

Some comments added to hb-ft.h, hoping to make future clients make better
choices.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=75299