[waffle] json, approach 2, version 2

[Frank Henigman]

Thanks Emil and Chad for reviewing my json series.  All suggestions
implemented in v2, except where I replied inline.  I'll hold off
sending in case there's more back-and-forth over the first set of
comments.  Would also be nice if Chad merged his get-current branch
into master, as I use it in v2.

When comparing my json output to the landed json output I noticed that
the landed version omits the context flags found in the old format.
Was that deliberate?  If so I'll remove it from my json.

Not sure if I did the right thing with glx info.  Seems like all three
sections (server, client, common) show about the same list of
extensions.  That can wait until I send v2, or if anyone wants to look
now:
https://github.com/fjhenigman/waffle/commit/b358ac50c00ce51fae6546b1e96c9adc32fcbdc7