Strict #include dependency ordering

[rickyjames35]

There seems to be a required order of #includes when using openxr_platform.h. For example, this compiles:

#define XR_USE_PLATFORM_ANDROID
#define XR_USE_GRAPHICS_API_VULKAN
#include <vulkan/vulkan.h>
#include <openxr/openxr_platform.h>

This does not:

#define XR_USE_PLATFORM_ANDROID
#define XR_USE_GRAPHICS_API_VULKAN
#include <openxr/openxr_platform.h>
#include <vulkan/vulkan.h>

You will see many errors about unknown Vulkan types like so....

error: unknown type name 'VkFormat'

The reason for this is openxr_platform.h has a dependency on vulkan.h but does not #include it. It's generally considered best practice to #include or forward declare dependencies directly in the header that's using them so the burden isn't on the user to infer the proper order of includes. There is some good info in here for anyone new to this concept https://stackoverflow.com/questions/2762568/c-c-include-header-file-order

I would like to understand Khronos's philosophy on this. Also I may just be incorrectly adding OpenXR as a dependency to my project. However looking though https://github.com/KhronosGroup/OpenXR-Tutorials and the hello_xr test it seems they also have a strict ordering of the includes.