This prepares the headers for exporting symbols using visibility
attributes or __declspec(dllexport), so that we do not need to maintain
symbols listing files, as this is what was and is done in GLib and GTK+.

This fixes a design bug with sanitize and sub-blobs that can
cause crashes.  Jonathan and I found and debugged this issue
when we tested a corrupt font with the md5sum / filename:
ea395483d37af0cb933f40689ff7b60a.  Two hours of intense
debugging we found out that the font has overlapping GSUB/GPOS
tables, and as such, sanitizing the second table can modify
the first one, which can cause all kinds of undefined behavior.

The correct way to fix this is to make sure sub-blobs are
always created readonly, since we consider the parent blob
to be a shared resource and can't modify it, even if it *is*
writable.

This essentially makes the READONLY_MAY_MAKE_WRITABLE mode
unused...  Maybe we should simply remove / deprecate it.

As was reported to me, glib-mkenum does not understand named enums,
so remove for now.

gdb was showing <anonymous enum> instead of useful stuff, so name
all our enums.

Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly.  This makes it easier to
refactor headers later on without breaking compatibility.

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

This was a bizzare piece of API that I inherited from cairo.  It has
been wrong adding them to cairo in the first place.  Remove them before
someone uses them!

This is the common convention for language binding tools.

Unlike the rest of the memory-mode enum, this one didn't only
describe the access mode of the input memory region.  Remove it.

If someone wants to inhibit duplicating, they can lock the blob
and throw away the key.

Based on mailing list discussion with Carl Worth.

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.