Consider black border around visible display area

[tomcw]

Motivations are:

1. With the VidHD card selected: this already adds a black border to the window, so consider simplifying things by just adopting this border always (ie. with or without the VidHD card).

2. Also there's a "hack" in NTSC.cpp such that MIXED (eg) HGR+80COL aligns the top and bottom halves, when really the 14M mode (80COL) starts 7 pixels earlier, eg. https://github.com/AppleWin/AppleWin/issues/555#issuecomment-412345857. So with a border, we can remove this video hack, to again simplify the code.

Other things to consider:

• add this border as an option?
  • but what's the point of this option? as the code will become more complicated overall, not simpler.
• how will screenshot work? (eg. PrintScrn to generate a BMP)
  • perhaps for non-MIXED HGR+80COL, then just output the borderless 560x384 image (as we currently do); and for the MIXED cases, then auto-align the 2 halves.

[Michaelangel007]

I AM in favor of a black border for aesthetic reason as long as it is OPT-IN.

My concern about adding a border deals with screenshot manipulation. Users have existing workflows (ImageMagick, GIMP, Photoshop, etc.) that process images and it would be extremely rude to break that flow.

If we add a black border the screenshot filename probably should reflect that. Game_border_00000000.bmp if the border is on else the regular filename if off such as Game_00000000.bmp so users aren't surprised and send us a nasty (and quite justified) bug report asking why we changed the default behavior.