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: https://gitlab.freedesktop.org/wayland/weston/issues/278

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

include/libweston/backend-headless.h

@@ -41,6 +41,9 @@ struct weston_headless_backend_config {
 
 	/** Whether to use the pixman renderer, default is no-op */
 	bool use_pixman;
+
+	/** Whether to use the GL renderer, conflicts with use_pixman */
+	bool use_gl;
 };
 
 #ifdef  __cplusplus