MTLRenderCommandEncoder creation will be deferred until a render pass
ends.
Commands will be stored into a back-end owned buffer during render pass.
At the end of the render pass, those commands will be re-encoded into an
actual MTLRenderCommandEncoder.

Benefits:
- Useful for future implementation of occlusion query buffer where it
  could be allocated right before the end of a render pass to be big
  enough to store all queries within the render pass.
- It's possible to change load option (deferred clear) as long as there
  is no draw call issued yet. This is not implemented yet.
- Possibility of commands' re-ordering.

Bug: angleproject:2634
Change-Id: I1348716aa882c0540d9120bf175d8dac13fb58bd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2193196
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>