add GLES3 headers from Khronos for OpenVR video driver.

src/video/khronos/GLES3/gl3platform.h

@@ -0,0 +1,27 @@
+#ifndef __gl3platform_h_
+#define __gl3platform_h_
+
+/*
+** Copyright 2017-2020 The Khronos Group Inc.
+** SPDX-License-Identifier: Apache-2.0
+*/
+
+/* Platform-specific types and definitions for OpenGL ES 3.X  gl3.h
+ *
+ * Adopters may modify khrplatform.h and this file to suit their platform.
+ * Please contribute modifications back to Khronos as pull requests on the
+ * public github repository:
+ *      https://github.com/KhronosGroup/OpenGL-Registry
+ */
+
+#include <KHR/khrplatform.h>
+
+#ifndef GL_APICALL
+#define GL_APICALL  KHRONOS_APICALL
+#endif
+
+#ifndef GL_APIENTRY
+#define GL_APIENTRY KHRONOS_APIENTRY
+#endif
+
+#endif /* __gl3platform_h_ */