This change creates a test only implementation of flutter::Surface backed by an
offscreen Vulkan GrContext. Much of the code in this test Surface was lifted
from flutter::VulkanWindow which I was unable to use without extricating it
from the VkSurface/VkSwapchain code which we do not want to use in offscreen
tests. I would recommend refactoring VulkanWindow to separate GrContext
creation and VkSwapchain creation in order to promote greater code reuse
between onscreen and offscreen vulkan paths.

This change is excersised thoroughly by the shell tests and was manually
tested against these tests on Fuchsia on Intel.