Heavy-handed implementation of implicit or explicit VkSubpassDependency
to VK_SUBPASS_EXTERNAL after a subpass.

Fixes various test flake.

Test: dEQP-VK.glsl.derivate.*
Change-Id: I9cb60828e6aff344e28dd7a8e1a867b5c8a11c22
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27708Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>