Currently stack frames are still WIP (inlined at is still unimplemented). The current implementation can leak stack frames, which collect between shader invocations.

Ensure the stack frame depth is the same on exit as it was on entry.

Bug: b/148401179
Change-Id: I95fb0739dd9691942826cf22eb097708762b72e5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44289
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>