Add GDK_GL env var and GdkGLFlags

Add GDK_GL env var and GdkGLFlags

This moves the GDK_ALWAYS_USE_GL env var to GDK_GL=always.
It also changes GDK_DEBUG=nogl to GDK_GL=disable, as GDK_DEBUG
is really only about debug loggin.

It also adds some completely new flags:

 software-draw-gl:
   Always use software fallback for drawing gl content to a cairo_t.
   This disables the fastpaths that exist for drawing directly to
   a window and instead reads back the pixels into a cairo image
   surface.

 software-draw-surface:
   Always use software fallback for drawing cairo surfaces onto a
   gl-using window.  This disables e.g. texture-from-pixmap on X11.

 software-draw:
   Enables both the above.