Previously ProgramMtl could try to bind fixed slots to samplers based on
layout (set=..., binding=...). However, GLSL layout model is different
from Metal slots assignment. For example, The following is valid layout
in GLSL:
- array samplers A[2] is bound to index 0.
- array samplers B[2] is bound to index 1.
It is invalid to do so in Metal, since A occupies 2 slots, thus samplers
  B[2] must be bound to slots starting from 2.

New binding method: let spirv-cross auto assigns the texture slots and
  retrieve them after compilation.

Incomplete textures moved to ContextMtl using IncompleteTextureSet.

New test added: GLSLTest.ArrayOfStructContainingArrayOfSamplers.

Bug: angleproject:2634
Change-Id: Ib0edaaf8b20512e1272c37c1d4b16a88a5b35e75
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2193193
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>