vulkan/screenshot-layer: Add Vulkan screenshot layer

What does this MR do and why?

vulkan/screenshot-layer: Add Vulkan screenshot layer

This change adds a Vulkan screenshot layer that allows users to take
screenshots from a Vulkan application, but has an emphasis on
performance, decreasing the performance impact on the application
involved. This allows for automated setups to use this layer to take
screenshots for navigating various in-application menus.

This layer works by hooking into various common Vulkan setup functions, until
it enters the vkQueuePresentKHR function, and from there it copies the current
frame's image from the swapchain as an RGB image to host-cached memory, where
we will receive the information as a framebuffer pointer. From there, we copy
the framebuffer contents to a thread that will detach from the main process
so it can write the image to a PNG file without holding back the main thread.

This layer was created from using the existing overlay layer as a template,
then adding portions of LunarG's VulkanTools screenshot layer:
https://github.com/LunarG/VulkanTools/blob/main/layersvt/screenshot.cpp

More specifically, there were usages of functions, along with modifications of
various functions from screenshot.cpp in the VulkanTools project, used in
screenshot.cpp.

There are some sections of the screenshotting functionality that remain
unmodified from the original screenshot.cpp file in VulkanTools, including the
global locking structures and the writeFile() function, which takes care of
obtaining the images from the swapchain. There were various areas in which
modifications were made, including how images are written to a file (using PNG
instead of PPM, introducing threading, added fences/semaphores, etc), along
with many smaller changes.

Signed-off-by: Casey Bowman <casey.g.bowman@intel.com>