GDK defaults to asking for an OpenGL 3.2 Core Profile, but if we get a
legacy profile from the underlying windowing system, the OpenGL version
will be fixed to 3.0. If that happens, we need to set the legacy bit on
the GdkGLContext, since that bit will be used to determine the version
and type of GLSL shaders that will be used by application and toolkit
code alike.

(cherry picked from commit 31c05771e957f2f54fb197a391b63e6ef3394a26)
Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>