Minor optimization
Blur & Be do not matter whether border and body overlap
Remove outdated comment
Bugfix: time shift by half frame
Add several reg options for subtitle frames dumping: dump_subtitle_frame_start_time & dump_subtitle_frame_end_time set the time range of subtitle frames to dump; dump_subtitle_frame_to_txt & dump_subtitle_frame_alpha_channel & dump_subtitle_frame_rgb_bmp set the output files; dump_subtitle_frame_background_color sets the background color used when dump RGB to bmp.
Minor enhancement of show bitmap box function: the box color can be specified now.