Fix for fuzzer-discovered error in SPIR-V with RTFlip.

SPIR-V code generation synthesizes some extra variables that don't
actually exist in the Program. Checking the ProgramUsage of these
variables would fail; ProgramUsage::get doesn't know about these
variables, so it asserts (and would consider them as dead even if it
didn't assert). We now track our SPIR-V bonus variables in a separate
set, and always report them as live.

Change-Id: If2f681470654025abf7ca4b3ec8126de2eb01297
Bug: oss-fuzz:36770
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/435625
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed