[type42] Remove unused member extension_component
[threadsafety] Allocate AF_Loader on the stack instead of AF_Module

Stop sharing a global AF_Loader.  Allocate one on the stack during
glyph load.

Right now results in about 25% slowdown.  However, I don't fully
understand why that should be as all the init/done really does is
clearing memory.  Will investigate more.

With this patch loading glyphs from different faces from different
threads doesn't immediately crash in the autohinting loader code.

Bugs:
https://bugzilla.redhat.com/show_bug.cgi?id=1164941
[threadsafety] Allocate TT_ExecContext in TT_Size instead of TT_Driver

Previously the code had stipulation for using a per-TT_Size exec
context if size->debug was true.  But there was no way that size->debug
could *ever* be true.  As such, the code was always using the singleton
TT_ExecContext that was stored in TT_Driver.  This was, clearly, not
threadsafe.

Remove size->debug.

Remove TT_Driver-global TT_ExecContext.

Lazy-create a TT_ExecContext for the TT_Size when needed.

With this patch loading glyphs from different faces from different
threads doesn't crash in the bytecode loader code.
[threadsafety] Allocate render pool for smooth rasterizer on the stack

Instead of using the render_pool member of FT_Library that is provided down
to the raterizer, completely ignore that and allocate needed objects on the
stack instead.

With this patch rasterizing glyphs from different faces from different threads
doesn't crash in the smooth rasterizer.

Bugs:
https://bugzilla.redhat.com/show_bug.cgi?id=678397
https://bugzilla.redhat.com/show_bug.cgi?id=1004315
https://bugs.freedesktop.org/show_bug.cgi?id=69034
https://bugzilla.redhat.com/show_bug.cgi?id=1165471