egl/wayland: implement SwapInterval > 1

Currently the EGL wayland platform is maximally unhelpful to applications that want to draw at less than the full display refresh rate. Here is my naive implementation of SwapInterval > 1, which seems to work.

However, I'm not enough of a wayland spec lawyer to tell if this is actually safe to do, especially since a (now removed) comment stated "that the only way we be sure to get a frame callback is to attach a new buffer", which I'm certainly not doing here. Maybe @daniels, @emersion or @pq could take a look and tell me if what I'm doing is supported or a blatant abuse of the protocol.