Reland "Metal: Hold refs for input buffers from bindBuffer calls."

This is a reland of a75dc33a677f130df1b24855df2f30fccb2e5137

Original change's description:
> 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>

Change-Id: I26ebad195fe044c82ea9ad5684ef1ac6d03cbc37
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306537
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
7 files changed