[subset] optimize glyph closure method: step1

Previous GSUB glyph closure is done by recursively visiting
all reachable lookup tables that apply to any glyphs in the
current/input glyph set, but actually only the glyphs that
the parent lookup can apply the recursion to can participate
in recursing the lookup. This is step 1 for glyph closure
optimization:
1. Add stack of currently active glyph set pointers into hb_closure_context_t
2. Update closure() method in simple GSUB tables to use
this stack in order not to change current glyph set at different stages
of recursion
3. Add function may_have_non_1to1() to GSUB tables