Consider the right CONTEXT_LOST token on OpenGL ES contexts

The KHR_robustness extension defined the following token as
returned by GetError (see spec at [1]):

  CONTEXT_LOST  0x0507

As noted in the spec, this token must have a "_KHR" suffix in
an OpenGL ES context, which is not being considered here, thus
making this type of builds fail.

[1] https://www.khronos.org/registry/OpenGL/extensions/KHR/KHR_robustness.txt

https://bugzilla.gnome.org/show_bug.cgi?id=781398

cogl/cogl/driver/gl/cogl-util-gl-private.h

@@ -37,6 +37,11 @@
 #include "cogl-gl-header.h"
 #include "cogl-texture.h"
 
+/* In OpenGL ES context, GL_CONTEXT_LOST has a _KHR prefix */
+#ifndef GL_CONTEXT_LOST
+#define GL_CONTEXT_LOST GL_CONTEXT_LOST_KHR
+#endif
+
 #ifdef COGL_GL_DEBUG
 
 const char *