clients/simple-egl: Use INT32_MAX for opaque region

Setting the opaque region correctly is common source of error for
clients that simply want to express that a whole surface is opaque.
This is especially true once buffer_scale and buffer_transform come
into play, as unlike for damage, where buffer_damage is the
encouraged and user friendly way today, opaque regions are always
in logical coordinates.

As faulty opaque regions don't have a visual impact in these cases
but only increase resource consumption, these errors often remain
for long times. See
gstreamer/gstreamer@1e2bc681


for one of many examples.

Give an easy example how to set the opaque region in a conformant
and reliable way.

Signed-off-by: Robert Mader <robert.mader@collabora.com>