[depend] Retain active glyph set storage

Dependency recursion moves each position glyph set onto the active stack.
Popping the stack then destroys the set and its backing pages, so the next
edge has to allocate them again.

Keep constructed stack slots and track the active depth separately. Reused
slots are cleared before use, preserving the previous empty-set semantics
while retaining their allocations across sibling traversals.

After contextual scratch pooling, this reduces Noto Nastaliq dependency
compilation from about 294ms to 277ms. Allocation calls fall from 1.19
million to 371 thousand, and cumulative allocated bytes fall from 163MB
to 80MB. Serialized output remains byte-identical.

Tests: meson test -C build (272/272)

Assisted-by: OpenAI Codex <codex@openai.com>

src/hb-ot-layout-gsubgpos.hh

diff --git a/src/hb-ot-layout-gsubgpos.hh b/src/hb-ot-layout-gsubgpos.hh
index ea06aa0..54cc393 100644
--- a/src/hb-ot-layout-gsubgpos.hh
+++ b/src/hb-ot-layout-gsubgpos.hh
@@ -484,25 +484,29 @@
 
   const hb_set_t& parent_active_glyphs ()
   {
-    if (!active_glyphs_stack)
+    if (!active_glyphs_stack_depth)
       return *glyphs;
 
-    return active_glyphs_stack.tail ();
+    return active_glyphs_stack[active_glyphs_stack_depth - 1];
   }
 
   hb_set_t* push_cur_active_glyphs ()
   {
-    if (unlikely (!active_glyphs_stack.push_or_fail ()))
+    if (active_glyphs_stack_depth == active_glyphs_stack.length &&
+	unlikely (!active_glyphs_stack.push_or_fail ()))
       return nullptr;
-    return &active_glyphs_stack.tail ();
+
+    hb_set_t *set = &active_glyphs_stack[active_glyphs_stack_depth++];
+    set->clear ();
+    return set;
   }
 
   bool pop_cur_done_glyphs ()
   {
-    if (!active_glyphs_stack)
+    if (!active_glyphs_stack_depth)
       return false;
 
-    active_glyphs_stack.pop ();
+    active_glyphs_stack_depth--;
     return true;
   }
 
@@ -627,6 +631,7 @@
   hb_face_t *face;
   hb_set_t *glyphs;
   hb_vector_t<hb_set_t> active_glyphs_stack;
+  unsigned active_glyphs_stack_depth = 0;
   recurse_func_t recurse_func;
   hb_codepoint_t lookup_index = HB_CODEPOINT_INVALID;
   hb_set_t lookups_seen;