Add a new function to TextureBuilder that takes a GLsync that
requires internal code to wait on before using the texture.
Somewhat sneakily, we don't take the sync if syncs are not supported by
the current GL context.
As public API has no code to query the sync for the destroy notify, this
is fine and it means we don't have to do the check every time we want to
call gdk_texture_get_sync() internally.
Building GL textures is complicated, so create an object to make them.
So far, this object just contains the functionality of
gdk_gl_texture_new(), but that will change in the future.
