exposay: make inner border dependent of exposay's surfaces size

exposay: make inner border dependent of exposay's surfaces size

We've been using an inner border of fixed size (80px), but this
is dangerous. If you have too many open applications or a small
window, the surface size computed will be negative, crashing
the exposay: "error: weston_view transformation not invertible".

Also, it creates a lot of unnecessary space, making the exposay
unusable when we have a small window or many applications open.

Make inner border to be 10% of surface size and surface size to
be 90% of its original size, avoiding the crashes and making it
more visually pleasant.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>