ext_external_objects/vk-image-display-overwrite: new interoperability test

A new interoperability test where an image is allocated by Vulkan and is filled with bands of different colors. OpenGL uses that image as a texture and overwrites one band. Vulkan maps the memory to take back the pixels. As piglit can't display Vulkan images, a new OpenGL texture is created and filled with the pixels that were read from that mapped Vulkan memory. We probe their values to see if they match the colors with which GL had overwritten the Vulkan ones.