Coroutines are emulated by using fibers.

Bug: b/145754674
Change-Id: I3f4bf29d26a75a2386ed812dd821d8a7a8276305
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40548Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>