subsurface: Calculate global coordinates with all ancestors

subsurface: Calculate global coordinates with all ancestors

The 'x' and 'y' member variables of E_Client for a subsurface are only
updated when its subsurface position needs to be updated. In other
words, if there's no update made on the position using
wl_subsurface.set_position, they would remain at their default values,
which are zero.

Therefore, we need to consider calculating the global coordinates of the
subsurface considering the positions of all its ancestors.

Change-Id: I92b49e1ca5b90bd1ee4ef43a058b7495a39796aa