Vivante drivers use the VK_KHR_display extension for rendering directly
to the frame buffer. This patch adds support to the video driver for
Vulkan rendering using that method.
- Add an utility function SDL_Vulkan_Display_CreateSurface that creates
a surface using this extension. The display to use (if there are
multiple) can be overridden using the environment variable
"SDL_VULKAN_DISPLAY".
- Use this function in a new compilation unit SDL_vivantevideo.c,
which implements the SDL_VIDEO_VULKAN methods of the driver structure.
