Move native stub initialization from extgl into GLContext.java. Makes it much easier to

exlude extensions classes. Furthermore, GLContext.useContext(null) can now be used to
reset the native stubs when no context is current.

src/java/org/lwjgl/opengl/ARBProgram.java

@@ -129,6 +129,8 @@ public class ARBProgram {
 	public static final int GL_MATRIX31_ARB = 0x88DF;
 
 
+	static native void initNativeStubs();
+
 	// ---------------------------
 	public static void glProgramStringARB(int target, int format, ByteBuffer string) {
 		nglProgramStringARB(target, format, string.remaining(), string, string.position());