compositor: replace weston_buffer_viewport::viewport_set

compositor: replace weston_buffer_viewport::viewport_set

Remove the explicit boolean variable, and use illegal width to denote
"not set".

Split the boolean into two, so we can later start having buffer.src_*
and surface.* set or not set independently. This may become useful when
the wl_viewport interface is changed to allow modifying them separately.

At the moment, both buffer.src_width and surface.width conditions are
always in sync.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>