compositor: change rounding in weston_surface_to_buffer_rect()

Rounding both corners of the rectangle down can result in a 0
width/height rectangle before passing to weston_transformed_rect.

This showed up as missing damage in weston-simple-damage (the
bouncing ball would leave green trails when --use-viewport was
used)

Reviewed-by: Daniel Stone <daniels@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>