Integrate AMD FSR2 backend and document SDK bootstrap

include/third_party/ffx_fsr2_compat.h

@@ -0,0 +1,15 @@
+#pragma once
+
+// Compatibility shim for building AMD FSR2 SDK sources on non-MSVC toolchains.
+#include <stddef.h>
+#include <wchar.h>
+#include <locale>
+#include <codecvt>
+
+#ifndef _countof
+#define _countof(x) (sizeof(x) / sizeof((x)[0]))
+#endif
+
+#ifndef wcscpy_s
+#define wcscpy_s(dst, src) wcscpy((dst), (src))
+#endif