[webkit-gtk] Add API for disabling the accelerated compositing

[Michael Catanzaro]

On Tue, 2016-11-15 at 21:41 +1100, Michael Gratton wrote:
> Dumb question, but would a workaround for the moment be to set the 
> WEBKIT_DISABLE_COMPOSITING_MODE env var from the embedding process 
> itself, before the WK2 context is initialised, so that AC is 
> subsequently disabled?

At the very very top of main, before any calls that could possibly
create a secondary thread. I'm tired of seeing GNOME stuff crash due to
use setenv() in multithreaded programs. :)

Michael