update GLAD to allow OpenGL 4.6 and below (#379)

* update glad to allow up to OpenGL 4.6 Core

* GLAD now defines NOMINMAX and WIN32_LEAN_AND_MEAN

ext/glad/include/KHR/khrplatform.h

@@ -2,7 +2,7 @@
 #define __khrplatform_h_
 
 /*
-** Copyright (c) 2008-2009 The Khronos Group Inc.
+** Copyright (c) 2008-2018 The Khronos Group Inc.
 **
 ** Permission is hereby granted, free of charge, to any person obtaining a
 ** copy of this software and/or associated documentation files (the
@@ -26,18 +26,16 @@
 
 /* Khronos platform-specific types and definitions.
  *
- * $Revision: 32517 $ on $Date: 2016-03-11 02:41:19 -0800 (Fri, 11 Mar 2016) $
+ * The master copy of khrplatform.h is maintained in the Khronos EGL
+ * Registry repository at https://github.com/KhronosGroup/EGL-Registry
+ * The last semantic modification to khrplatform.h was at commit ID:
+ *      67a3e0864c2d75ea5287b9f3d2eb74a745936692
  *
  * Adopters may modify this file to suit their platform. Adopters are
  * encouraged to submit platform specific modifications to the Khronos
  * group so that they can be included in future versions of this file.
- * Please submit changes by sending them to the public Khronos Bugzilla
- * (http://khronos.org/bugzilla) by filing a bug against product
- * "Khronos (general)" component "Registry".
- *
- * A predefined template which fills in some of the bug fields can be
- * reached using http://tinyurl.com/khrplatform-h-bugreport, but you
- * must create a Bugzilla login first.
+ * Please submit changes by filing pull requests or issues on
+ * the EGL Registry repository linked above.
  *
  *
  * See the Implementer's Guidelines for information about where this file
@@ -102,8 +100,7 @@
 #elif defined (__SYMBIAN32__)
 #   define KHRONOS_APICALL IMPORT_C
 #elif defined(__ANDROID__)
-#   include <sys/cdefs.h>
-#   define KHRONOS_APICALL __attribute__((visibility("default"))) __NDK_FPABI__
+#   define KHRONOS_APICALL __attribute__((visibility("default")))
 #else
 #   define KHRONOS_APICALL
 #endif