Metal: Hold refs for input buffers from bindBuffer calls.

Mirrors what we have in Vulkan and Direct3D.
Also adds command buffer tracking, again like Vulkan and Direct3D.

Change-Id: I2280d92274d81830aec7950afc64a0147e38c317
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/305396
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>