Vulkan: Add semaphores test to VulkanExternalImageTest Add a more substantial test that uses semaphores to VulkanExternalImageTest. It's still just a clear in GL, which exposed a bug that a staged clear wasn't be flushed by a call to glSignalSemaphoreEXT. Bug: angleproject:3289 Change-Id: Id938eaf2c8c20cb0ae9fb948fbfcf3448980b577 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2195684 Commit-Queue: Michael Spang <spang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>