ext_external_objects-vk-depth-display: New interoperability test that uses a Vulkan depth buffer in OpenGL

The purpose of the test is to reuse a Vulkan depth buffer from OpenGL. The test first draws something with Vulkan with the z buffer enabled in depth 0.33. The Vulkan z buffer is then imported in OpenGL, where it is used as the depth attachment of a framebuffer object. The color attachment of the same framebuffer object is first cleared to green. Then, it is used as render target to render a blue quad in depth 0.66. The final result must be: a blue quad with a smaller green quad in the middle, as the region that has depth 0.33 is hiding the blue quad.