xdg: Use wlr_xdg_surface_get_geometry() to get size

xdg: Use wlr_xdg_surface_get_geometry() to get size

This fixes an issue with havoc not having a valid size on map().

Investigation showed that xdg_surface->current.geometry is set only by
the xdg_surface::set_geometry protocol message, which is optional. If
set_geometry is not called, then we are supposed to compute the size
from the surface buffer(s). wlr_xdg_surface_get_geometry() already
accounts for this, so we just need to use wlr_xdg_surface_get_geometry()
instead of reading xdg_surface->current.geometry directly.