basesink: Don't nest prepare/render calls

basesink: Don't nest prepare/render calls

When the first buffer arrives, we endup calling:

  ->prepare()
    ->prepare()
    ->preroll()
  ->render()

This will likely confuse any element using this method. With this patch,
we ensure the preroll take place before the first render prepare() is
called. This will result in:

  ->prepare()
  ->preroll()
  ->prepare()
  ->render()

https://bugzilla.gnome.org/show_bug.cgi?id=772468