frame: Fix crash when clicking below titlebar with broken gtk themes

When a gtk theme uses larger shadows for the unfocused state than for
the focused one, this can cause a crash in meta_frame_left_click_event.
Since whether to call meta_frame_left_click_event is decided based on
the decoration size before focusing and the control that was clicked on
after focusing, this can result in an event handled in
meta_frame_left_click_event being on the client area.

Fixes #1668

Part-of: <!1748>
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