gdk: Add GDK_DEBUG=nogl

This is mostly useful for fallback testing.

I suppose if people want finer grained GL ability testing, they can use
Mesa environment variables to tune things.

gdk/gdkinternals.h

@@ -85,7 +85,8 @@ typedef enum {
   GDK_DEBUG_EVENTLOOP     = 1 << 10,
   GDK_DEBUG_FRAMES        = 1 << 11,
   GDK_DEBUG_SETTINGS      = 1 << 12,
-  GDK_DEBUG_OPENGL        = 1 << 13
+  GDK_DEBUG_NOGL          = 1 << 13,
+  GDK_DEBUG_OPENGL        = 1 << 14
 } GdkDebugFlag;
 
 typedef enum {