headless, gl-renderer: support pbuffer outputs

Use the surfaceless platform in the headless backend to initialize the
GL-renderer and create pbuffer outputs. This allows headless backend to use
GL-renderer, even hardware accelerated.

This paves way for exercising GL-renderer in CI and using the Weston test suite
to test hardware GL ES implementations.

Relates to: wayland/weston#278



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>