CMake: improved OpenXR SDK handling

* Check if the SDK has already been included
* Group targets under DiligentCore/ThirdParty/OpenXR

Author: TheMostDiligent

ThirdParty/CMakeLists.txt

@@ -171,8 +171,8 @@ if (WEBGPU_SUPPORTED)
     set_directory_root_folder("dawn" "DiligentCore/ThirdParty/dawn")
 endif()
 
-if(DILIGENT_USE_OPENXR)
-    add_subdirectory(OpenXR-SDK EXCLUDE_FROM_ALL)    
+if(DILIGENT_USE_OPENXR AND NOT (TARGET openxr_loader))
+    add_subdirectory(OpenXR-SDK EXCLUDE_FROM_ALL)
 endif()
 
 if ((${DILIGENT_BUILD_GOOGLE_TEST}) AND (NOT TARGET gtest))

ThirdParty/OpenXR-SDK/CMakeLists.txt

@@ -11,3 +11,4 @@ FetchContent_Declare(
     URL https://github.com/KhronosGroup/OpenXR-SDK-Source/archive/refs/tags/release-1.0.34.zip
 )
 FetchContent_MakeAvailable(OpenXR)
+set_directory_root_folder("${openxr_SOURCE_DIR}" "DiligentCore/ThirdParty/OpenXR")