Prefer composition over inheritance.
Makes it clear that the virtual destructors on these processors are unnecessary.

Bug: b/126126820
Change-Id: I062c06191ec1cdfb81f2a2773a976bf5c596e658
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43817Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>