[Metal] Support pointer SNode on Metal

[k-ye]

Related issue = #1174

[k-ye]

/format

[k-ye]

/format

[k-ye]

Hopefully this is in a stable enough point now:

• All tests involving pointer have passed on metal
• mgpcg.py, mgpcg_advanced.py produced the same result as CPU
• Got a small_particle_renderer.py working. The original one took too much memory. Because of the 32-bit data-type limitation, right now the Metal backend only supports a maximum of ~4GB memory pool.

small_particle_renderer.txt

---

• Fail loud when OOM
• This test failed because of how refine_coordinates works. Needs additional fix:

  taichi/tests/python/test_struct_for_intermediate.py

  Lines 4 to 22 in 04ea068

  	@ti.test(require=ti.extension.sparse, demote_dense_struct_fors=False)
  	def test_nested():
  	x = ti.field(ti.i32)
  	p, q = 3, 7
  	n, m = 2, 4
  	ti.root.dense(ti.ij, (p, q)).dense(ti.ij, (n, m)).place(x)
  	@ti.kernel
  	def iterate():
  	for i, j in x.parent():
  	x[i, j] += 1
  	iterate()
  	for i in range(p):
  	for j in range(q):
  	assert x[i * n, j * m] == 1