Fixes https://github.com/behdad/harfbuzz/issues/474

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1452#c5

CC https://github.com/behdad/harfbuzz/issues/139

Yay, works!

Fixes https://github.com/behdad/harfbuzz/pull/377

Should be all good now..

Almost there..

hb_font_get_glyph_extents now works for color bitmap fonts.
Currently only font having index format 1 and image format 17
is supported.

Ouch!

Fixes https://github.com/behdad/harfbuzz/issues/236
Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=627953

To make a release...

Apparently some clients have reference-table callbacks that copy the table.
As such, avoid loading 'glyf' table which is only needed if fallback positioning
happens.

New API:
- hb_font_get_nominal_glyph_func_t
- hb_font_get_variation_glyph_func_t
- hb_font_funcs_set_nominal_glyph_func()
- hb_font_funcs_set_variation_glyph_func()
- hb_font_get_nominal_glyph()
- hb_font_get_variation_glyph()

Deprecated API:
- hb_font_get_glyph_func_t
- hb_font_funcs_set_glyph_func()

Clients that implement their own font-funcs are encouraged to replace
their get_glyph() implementation with a get_nominal_glyph() and
get_variation_glyph() pair.  The variation version can assume that
variation_selector argument is not zero.

Exactly the same problem that I fixed in
63ef0b41

I rewrote the table checking yesterday in
67f8821f
and introduced the exact same issue again. :(
Good thing we have ongoing fuzzing going now.  Was discovered
immediately by libFuzzer.  Thanks kcc!

https://github.com/behdad/harfbuzz/issues/139#issuecomment-153449473
Fixes https://github.com/behdad/harfbuzz/issues/156

Also route fuzzing-related tests through hb-ot-font, to reduce dependency
on FreeType behavior for badly-broken fonts.  Fixes failing test with
FreeType master.

Discovered by libFuzzer.  Ouch!

https://github.com/behdad/harfbuzz/issues/139#issuecomment-148289957

https://github.com/behdad/harfbuzz/issues/139#issuecomment-147616887

This brings ot-fonts into almost-complete shape and mostly in par with
ft font.