Use placement new for PtexHashMap TableHeader and Entry objects

Placement new is required by C++ to see objects allocated with malloc as
objects and not just raw memory. Doing so allows the compiler to
properly analyze object lifetimes and usage.

Without the placement new, and with dead code / dead store elimination
enabled, an unexplained crash was occuring with GCC 15 when running the
regression tests. With this change, the crash no longer appears.

Author: Brent Burley

src/ptex/PtexHashMap.h

@@ -264,9 +264,14 @@ class PtexHashMap
         memsize = sizeof(TableHeader) + sizeof(Entry) * numEntries;
         void* table = malloc(memsize);
         memset(table, 0, memsize);
-        TableHeader* header = (TableHeader*) table;
+        TableHeader* header = new (table) TableHeader;
         header->numEntries = numEntries;
         header->size = 0;
+        Entry* entries = (Entry*)((char*)table + sizeof(TableHeader));
+        for (int32_t i = 0; i < numEntries; ++i)
+        {
+            new (&entries[i]) Entry();
+        }
         return table;
     }