Control legacy GL context via environment variable

For testing purposes, we may want to force the creation of legacy GL
contexts via an environment variable.

https://bugzilla.gnome.org/show_bug.cgi?id=756142

gdk/gdkinternals.h

@@ -99,7 +99,8 @@ typedef enum {
   GDK_GL_ALWAYS                 = 1 << 1,
   GDK_GL_SOFTWARE_DRAW_GL       = 1 << 2,
   GDK_GL_SOFTWARE_DRAW_SURFACE  = 1 << 3,
-  GDK_GL_TEXTURE_RECTANGLE      = 1 << 4
+  GDK_GL_TEXTURE_RECTANGLE      = 1 << 4,
+  GDK_GL_LEGACY                 = 1 << 5
 } GdkGLFlags;
 
 extern GList            *_gdk_default_filters;