LibGL: Implement glGetError and underlying function

This implements `glGetError` and correctly sets the state machine's
error macro (similar to LibC `errno`) when an invalid operation is
performed. This is reset on completion of a successful operation.

Userland/Libraries/LibGL/GLUtils.cpp

@@ -29,3 +29,8 @@ void glViewport(GLint x, GLint y, GLsizei width, GLsizei height)
 {
     g_gl_context->gl_viewport(x, y, width, height);
 }
+
+GLenum glGetError()
+{
+    return g_gl_context->gl_get_error();
+}