compositor: use floats in computing bounding box

compositor: use floats in computing bounding box

In surface_compute_bbox(), call surface_to_global_float() instead of
weston_surface_to_global(). This avoids the recursion:

weston_surface_update_transform()
  weston_surface_update_transform_enable()
    surface_compute_bbox()
       weston_surface_to_global()
         weston_surface_update_transform()

which might be non-obvious when reading the code.

Computing the min and max coordinates in floats, we can have a tight
rounding margin by using floor() and ceil().

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>