Previously Frame was an Thread-internally managed structure, with special accessors for the variable containers of the top most frame.

With this change, the Frame can be modified by calling `Thread::update()`. This allows for more powerful edits of the frame, while still preserving a mutex lock over the structure.

Bug: b/145351270
Change-Id: I81e87884e26bdd6f22b47e8f71b37bbfc91c7830
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39882
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>