egl: store the native surface pointer in struct _egl_surface

egl: store the native surface pointer in struct _egl_surface

Each platform stores this in a different place:
  - platform_drm uses dri2_surf->gbm_surf->base
  - platform_android uses dri2_surf->window
  - platform_wayland uses dri2_surf->wl_win
  - platform_x11 uses dri2_surf->drawable
  - platform_x11_dri3 uses dri3_surf->loader_drawable.drawable
  - haiku doesn't even store it!

We need access to the native surface since the specification asks us
to refuse creating a new surface if there's already an EGLSurface
associated with native_surface.

An alternative to this patch would be to create a new
API.GetNativeWindow callback that each platform would have to
implement. While that's something we can definitely do, I prefer
this approach.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>