Unused InterfaceBlocks were not added to the ProgramUsage map. The
ProgramUsageVisitor now makes sure to account for them during its
initial scan.
Change-Id: If3afac8e954c5b685ddc6b63b0f771d8c0b8f207
Bug: oss-fuzz:33405
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/398016
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
