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 (closed)