EGL_KHR_surfaceless_context extension allows contexts to be made current
without a default winsys fbo. This extension specifies what ES 1.1 and
2.0 should do (the ES 3.0 spec already does).
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com> Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>