Fixes https://github.com/harfbuzz/harfbuzz/issues/653

Sorry for the noise, downstream custom builders.  Please adjust.

As discussed on
https://github.com/behdad/harfbuzz/pull/10#issuecomment-31442030

See thread "an issue regarding discrepancy between Korean and Unicode
standards" on the mailing list for the rationale.  In short: Uniscribe
doesn't, so fonts are designed to work without it.

So we can handle Sinhala split matras smartly...  Coming soon.

Had to do some refactoring to make this happen...

Under uniscribe bug compatibility mode, we still plit them
Uniscrie-style, but Jonathan and I convinced ourselves that there is no
harm doing this the Unicode way.  This change makes that happen, and
unbreaks free Sinhala fonts.

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.

Essentially move the glyph mapping to normalization process.
The effect on Devanagari is small (but observable).  Should be more
observable in simple text, like ASCII.

...again.

Add a shaper class struct.

In preparation for Hangul shaper.

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.

Not complete yet, font selection doesn't work.  But hey it shapes!

This is not supposed to be a production backend, more like a testing
backend.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.