To be used for a variety of purposes.  We save up to five characters
in each direction.  No public API changes, everything is taken care
of already.  All clients need to do is to call hb_buffer_add_utf* with
the full text + segment info (or at least some context) instead of
just passing in the segment.

Various operations (hb_buffer_reset, hb_buffer_set_length,
hb_buffer_add*) automatically reset the relevant contexts.

Speeds up UTF-8 parsing by calling strlen().

Also move UTF routines into a separate file, to be reused from shapers
that need it.

And hb_buffer_content_type_t and enum values.

No panic, we reeally insert dotted circle when it's absolutely broken.

Fixes most of the dotted-circle cases against Uniscribe. (for Devanagari
fixes 80% of them, for Khmer 70%; the rest look like Uniscribe being
really bogus...)

I had to make a decision.  Apparently Uniscribe adds one dotted circle
to each broken character.  I tried that, but that goes wrong easily with
split matras.  So I made it add only one dotted circle to an entire
broken syllable tail.  As in: "if there was a dotted circle here, this
would have formed a correct cluster."  That works better for split
stuff, and I like it more.

To be used in places we access buffer vars...

It's considerably faster than the fallback implementation we had
previously!

This reorders glyphs within the cluster to a nominal order.  This should
have no visible effect on the output, but helps with testing, for
getting the same hb-shape output for visually-equal glyphs for each
cluster.

Detected by clang and lots of cmdline options.

Oops!

That's not in the OpenType spec, but it's what MS and Adobe do.

Patch from Kenichi Ishibashi.

For two reasons:

1. User can always call hb_buffer_pre_allocate() themselves, and

2. Now we do a pre_alloc in add_utfX anyway, so the total number of
reallocs is limited to a small number (~3) anyway.  This just makes the
API cleaner.

We do a conservative estimate of the number of characters, but still,
this limits the number of buffer reallocs to a small constant.

We need this to set data on objects safely without worrying that some
other thread unsets it by setting it at the same time.

Since we already switched to accepting -1 as 'zero-terminated'.

A -1 text_length means: zero-terminated string.
A -1 item_length means: to the end of string.

hb_shape() now accepts a shaper_options and a shaper_list argument.
Both can be set to NULL to emulate previous API.  And in most situations
they are expected to be set to NULL.

hb_shape() also returns a boolean for now.  If shaper_list is NULL, the
return value can be ignored.

shaper_options is ignored for now, but otherwise it should be a
NULL-terminated list of strings.

shaper_list is a NULL-terminated list of strings.  Currently recognized
strings are "ot" for native OpenType Layout implementation, "uniscribe"
for the Uniscribe backend, and "fallback" for the non-complex backend
(that will be implemented shortly).  The fallback backend never fails.

The env var HB_SHAPER_LIST is also parsed and honored.  It's a
colon-separated list of shaper names.  The fallback shaper is invoked if
none of the env-listed shapers succeed.

New API hb_buffer_guess_properties() added.

Where causing issues for people with MSVC.