This saves each object allocation 72 bytes.  Now object overhead is just
16 bytes (on x86) instead of 88 bytes. Neat.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1023

Right now only Old Italic is marked as such.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1000

Towards making blob more memory-allocation-friendly

We should hang those off somewhere else. For now, the unused ones can go.

X-terminated means don't access this, it's not located correctly.
Z-terminated means this is a C array with no bound checking.

Part of https://github.com/harfbuzz/harfbuzz/issues/1017

Part of https://github.com/harfbuzz/harfbuzz/issues/1017

Part of https://github.com/harfbuzz/harfbuzz/issues/1017

Part of https://github.com/harfbuzz/harfbuzz/issues/1017

For consistency.

Part of https://github.com/harfbuzz/harfbuzz/issues/1017

Fixes #779 and follow up to b01d9b3d

https://bugzilla.gnome.org/show_bug.cgi?id=659212 for more context

To revive Travis CI bot

It was ignoring the restriction that gids must be consecutive to be placed into a single group.

As Khaled's finding on #981 this is the way Uniscribe detects those fonts. This is its detection part.

Splitted from #950 and #986 IIRC Chromium had a policy about this encouraging it,
not sure about automated way to detect and add them but for now lets have the needed
ones of them.