Enforce that layout(binding=...) is set on interface blocks in Metal.

Previously, we would emit an invalid [[buffer(-1)]] annotation on the
block, causing the Metal compilation to fail.

Change-Id: I68b2439c05db3163686e84c5dcc9a5c43870ff67
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340761
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
16 files changed