Instead of destroying sources directly while freeing the context, and
potentially freeing them if this was the last reference to them, collect
new references of all sources in a separate list before and at the same
time invalidate their context so that they can't access it anymore. Only
once all sources have their context invalidated, destroy them while
still keeping a reference to them. Once all sources are destroyed we get
rid of the additional references and free them if nothing else keeps a
reference to them anymore.

This fixes a regression introduced by 26056558 in 2012.

The previous code that invalidated the context of each source and then
destroyed it before going to the next source without keeping an
additional reference caused memory leaks or memory corruption depending
on the order of the sources in the sources lists.

If a source was destroyed it might happen that this was the last
reference to this source, and it would then be freed. This would cause
the finalize function to be called, which might destroy and unref
another source and potentially free it. This other source would then
either
- go through the normal free logic and change the intern linked list
  between the sources, while other sources that are unreffed as part of
  the main context freeing would not. As such the list would be in an
  inconsistent state and we might dereference freed memory.
- go through the normal destroy and free logic but because the context
  pointer was already invalidated it would simply mark the source as
  destroyed without actually removing it from the context. This would
  then cause a memory leak because the reference owned by the context is
  not freed.

Fixes https://github.com/gtk-rs/glib/issues/583 while still keeping
https://bugzilla.gnome.org/show_bug.cgi?id=661767 fixes.