[smooth] Fix stand-alone compilation.

* src/smooth/ftgrays.c (FT_BEGIN_STMNT, FT_END_STMNT): Define.
[autofit] Improve coverage handling.

* src/autofit/hbshim.c (af_get_coverage): Don't exclude glyphs
appearing in the GPOS table if we are processing the default
coverage.
[afblue.pl]: Minor improvements.

* src/tools/afblue.pl: Allow whitespace before comments.
Ignore whitespace in strings.
Documentation fix.
Make `FT_Get_SubGlyph_Info' actually work.

* src/base/ftobjs.c (FT_Get_SubGlyph_Info): Return FT_Err_Ok
if there is no error.
[autofit] More coverage fixes for complex scripts.

* src/autofit/hbshim.c (af_get_coverage): Merge input glyph coverage
of GSUB lookups into output coverage.  Otherwise, ligatures are not
handled properly.
Don't check blue zone characters for default coverage.