plugins: add initial support for EGL.

plugins: add initial support for EGL.

Add initial support for EGL through GstVideoGLTextureUploadMeta.

Fix gst_vaapi_ensure_display() to allocate a GstVaapiDisplay off the
downstream supplied GstGLContext configuration, i.e. use its native
display handle to create a GstVaapiDisplay of type X11 or Wayland ;
and use the desired OpenGL API to allocate the GstVaapiDisplayEGL
wrapper.

https://bugzilla.gnome.org/show_bug.cgi?id=741079