[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
[raster] Remove 5-level gray AA mode from monochrome rasterizer

It was off by default and couldn't be turned on at runtime.
And the smooth rasterizer has had superceded it over ten years ago.
No point in keeping.  Comments suggested that it was there for
compatibility with FreeType 1.

550 lines down.
[threadsafety] Allocate render pool for mono 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 monochrome rasterizer.
[threadsafety] Do not reorder library->renderers upon use

Instead of keeping library->renderers in a MRU order, just leave
it as is.  The MRU machinery was an overengineering premature
optimization to begin with.  And wasn't thread-safe.

With this patch rasterizing glyphs from different faces from different
threads doesn't fail choosing rasterizer (FT_Err_Cannot_Render_Glyph).

Easiest to see that crash was to add a printf or otherwise let thread
yield in FT_Throw with debugging enabled.