Use triple buffer by default now on all platforms, remaing ones where:
* Mesa: seems to have been working well for a long time now, and not using
it gives issues with the latest Mesa 17.2.0.
* Windows software OpenGL: no longer supported since OpenGL 2.1 requirement
was introduced.
* OS X with thousands of colors: this option was removed in OS X 10.6, and
that's our minimum requirement.