gl-renderer: fix pbuffer surface creation

gl-renderer: fix pbuffer surface creation

When there's neither configless nor surfaceless EGL extension
(i.e. not a Mesa driver), Weston falls back to a dummy pbuffer surface.

Weston attempts to find for that surface an EGL config but uses a NULL
array of pixel formats. This fails with the following messages:

 EGL_KHR_surfaceless_context unavailable. Trying PbufferSurface
 Found an EGLConfig matching { pbf;  } but it is not usable because
    neither EGL_KHR_no_config_context nor EGL_MESA_configless_context
    are supported by EGL.
 failed to choose EGL config for PbufferSurface
 EGL error state: EGL_SUCCESS (0x3000)
 Failed to initialise the GL renderer;

Signed-off-by: Tomek Bury <tomek.bury@broadcom.com>