[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smooth rasterizer has had superceded it over ten years ago.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.

500 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.