Fix data race in PagedArray

[stuartcarnie]

This PR resolves a data race in the PagedArrayPool and PagedArray types. The thread sanitiser identified the data race via the page_pool field of the PagedArrayPool class. The write occurred:

godot/core/templates/paged_array.h

Line 62 in 9d1cbab

page_pool = (T **)memrealloc(page_pool, sizeof(T *) * pages_allocated);

and the concurrent read here:

godot/core/templates/paged_array.h

Line 78 in 9d1cbab

return page_pool[p_page_id];

The actual data race of the page_pool pointer being access concurrently is possibly safe, as it is a word read / write, however, an additional problem is the underlying memory in the memrealloc from the old allocation is being immediately zero'd in debug builds on macOS, so the call the get_page is sometimes returning an nullptr. In a release build, the old allocation may be immediately reused before the call to get_page, returning an invalid pointer, resulting in crashes, memory corruption, etc.

[RandomShaper]

Looks good besides the little nitpick.

[akien-mga]

Looks good! Could you squash the commits? See PR workflow for instructions.

[stuartcarnie]

@RandomShaper squashed

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉 -- hopefully the start of many, given what I know you've been cooking ;)

[akien-mga]

Cherry-picked for 4.2.2.