clients/simple-egl: Allow setting the swapinterval

eglSwapInterval() is intended to allow capping the frame rate to a divisor
of the display rate. This may not work if the GL library simple-egl is
using doesn't support it well.

It's still useful to test, so add a way to set it.

Signed-off-by: Derek Foreman <derekf.foreman@collabora.com>