Trim the bitmap's current width and height to be not more than its
parent. Otherwise, an automatic screen resize, such as triggered when
playing the robot briefings in Descent 2, will give the window a new
smaller bm_data, but continue to use the old dimensions. This then
causes load_briefing_screen -> gr_bitmap_scale_to -> scale_line to write
off the end of the smaller bm_data, causing memory corruption and
eventually a crash.
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