externals: Update glad to support OpenGL 4.6 compatibility profile

Now that we have an OpenGL compatibility profile we might want to use
OpenGL compatibility symbols that are not available in our current glad.

This commit has been generated with https://glad.dav1d.de/ with all
extensions enabled and OpenGL 4.6 compatibility profile.

externals/glad/include/KHR/khrplatform.h

@@ -90,12 +90,20 @@
  *                                  int arg2) KHRONOS_APIATTRIBUTES;
  */
 
+#if defined(__SCITECH_SNAP__) && !defined(KHRONOS_STATIC)
+#   define KHRONOS_STATIC 1
+#endif
+
 /*-------------------------------------------------------------------------
  * Definition of KHRONOS_APICALL
  *-------------------------------------------------------------------------
  * This precedes the return type of the function in the function prototype.
  */
-#if defined(_WIN32) && !defined(__SCITECH_SNAP__)
+#if defined(KHRONOS_STATIC)
+    /* If the preprocessor constant KHRONOS_STATIC is defined, make the
+     * header compatible with static linking. */
+#   define KHRONOS_APICALL
+#elif defined(_WIN32)
 #   define KHRONOS_APICALL __declspec(dllimport)
 #elif defined (__SYMBIAN32__)
 #   define KHRONOS_APICALL IMPORT_C
@@ -111,7 +119,7 @@
  * This follows the return type of the function  and precedes the function
  * name in the function prototype.
  */
-#if defined(_WIN32) && !defined(_WIN32_WCE) && !defined(__SCITECH_SNAP__)
+#if defined(_WIN32) && !defined(_WIN32_WCE) && !defined(KHRONOS_STATIC)
     /* Win32 but not WinCE */
 #   define KHRONOS_APIENTRY __stdcall
 #else