RFC: xserver update strategy in F21+

[Adam Jackson]

Since the modular X repackaging in FC5, we have limited X server updates
such that the ABI does not change.  F20 shipped with xserver 1.14.4, for
example, so we might update it to 1.14.7 but not to 1.15.0.  With the
reduced driver set in F21 it's now much more reasonable to push updates
to older releases as well.

With that in mind, I ask for feedback on how we'd actually like that to
work.  The kernel rebase policy seems like a pretty reasonable model:
F21 would stay on 1.16.x until there's an upstream 1.17.1 release, and
(if F20 were to be affected by this policy) F20 would wait until 1.17.1
had been tested in F21.

One thing we might have to play by ear is the interaction with binary
drivers.  The nvidia legacy driver, for instance, does not always have
builds available for arbitrarily new servers, which means updating the X
server might change you to an nvidia driver that no longer supports your
hardware.  Depending on how severe that cutoff is, it might be cause to
pin a particular Fedora release at a given server version.  I don't
think this is presently a problem, but it could be in the future.

This would also want some coordination with the various desktop
environments; the version of KDE in F21 might have latent bugs only
exposed by switching to F22's X, for example.  I have a reasonable idea
of how to test Gnome for that kind of thing, but for the others I'd need
some pointers.

So what do we think?  Good idea?  Bad idea?  Other things to watch for?

- ajax