add 'square pixel' aspect ratio option, add 640x400 res option, impro…

[rsn8887]

…ve image render quality for resolutions when pixels are not mapped 1:1 to Vita screen

• add 640x400 resolution option (specifically to display some games fullscreen without stretching)
• add aspect ratio option "square pixels," useful to achieve a larger display of some fullscreen games without distortion. For example, fullscreen games that run in 640x400 fit the Vita screen pretty well, even without stretching to 16:9.
• improve picture quality for resolutions where mac pixels do not map 1:1 onto vita pixels. Pixels are much less distorted now in those cases.

Note the OS resolution choice has higher priority than Basiliskii settings. So, to change resolution, it is not enough to only change it in the Basilisk prefs. The os will set it back during boot. It has to be changed also in the "monitors" extension in system 7.

[meetpatty]

Looks good, can we call the new option "Fit" as opposed to square pixels.

[rsn8887]

Done!