Fix child frame placement issues (bug#29953)

Fix child frame placement issues (bug#29953)

* src/nsterm.h (NS_PARENT_WINDOW_LEFT_POS):
(NS_PARENT_WINDOW_TOP_POS): Get the parent frame through the frame
struct as invisible child windows are detached from their parents in
NS.
* src/nsterm.m (x_set_offset): Offscreen frames have `nil' screen
value, so handle that gracefully.  Child frames with negative left and
top should be positioned relative to the bottom right of the parent
frame.