LibGL: Support missing context in glGetError and glGetIntegerv

In its current state, ScummVM seems to invoke these methods just after
destroying the current GL context. According to the OpenGL spec:

    "Issuing GL commands when the program does not have a current
     context results in undefined behavior, up to and including program
     termination."

Our old behavior was to deref a `nullptr`, which isn't that great. For
now, protect these two methods. If other ports seem to misbehave as
well, we can always expand the check to other methods.

Userland/Libraries/LibGL/GLContext.cpp

@@ -5,6 +5,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include "GLContext.h"
 #include "SoftwareGLContext.h"
 #include <LibGfx/Bitmap.h>