Add support for KTX & KTX2 image format

Add support glTF KHR_texture_basisu extension

thirdparty/libktx/utils/unused.h

@@ -0,0 +1,37 @@
+/* -*- tab-width: 4; -*- */
+/* vi: set sw=2 ts=4 expandtab: */
+
+/* Copyright 2019-2018 The Khronos Group Inc.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/* I'm extending this beyond the purpose implied by its name rather than creating
+ * a new file to hold the FALLTHROUGH declaration as this
+ * file is already included in most places FALLTHROUGH
+ * is needed.
+ */
+
+#ifndef _UNUSED_H
+#define _UNUSED_H
+
+#if (__cplusplus >= 201703L)
+#define MAYBE_UNUSED [[maybe_unused]]
+#elif __GNUC__ || __clang__
+  #define MAYBE_UNUSED __attribute__((unused))
+#else
+  // Boohoo. VC++ has no equivalent
+  #define MAYBE_UNUSED
+#endif
+
+#define U_ASSERT_ONLY MAYBE_UNUSED
+
+// For unused parameters of c functions. Portable.
+#define UNUSED(x) (void)(x)
+
+#if !__clang__ && __GNUC__ // grumble ... clang ... grumble
+#define FALLTHROUGH __attribute__((fallthrough))
+#else
+#define FALLTHROUGH
+#endif
+
+#endif /* UNUSED_H */