Display Preview Modes

* Not Yet Implemented (Sorry 8^)
• 'E' Extra-HalfBrite mode. A fixed, 'best-spread' 32/64 color palette is used. Works fairly well for images needing a variety of colors.
• 'G' Greyscale. 16 shades on OCS/ECS systems. 256 shades on AGA and Displaycard systems.
• 'W'*Window display under OS3. ENV variable 'POVRENDERSCREEN' can be used to select a public screen to open the display preview on, else default is used.
• 'M' A special mode that sends the pixel information to the GUI as a message packet, to be used as supported. The included GUI does not currently make use of this message, but may support external display modules in the future. Read GUI Communications about how to implement your own GUI, or even an AREXX front-end.
• 'H'*Hold-And-Modify mode. A fixed, 'best-spread' base palette is used. This module supports HAM6 (OCS/ECS) and HAM8 (AGA). If palette option of '8' or '6' is used, the appropriate mode will be forced. If no option, then HAM6 is used.
• 'P' Picasso96 support. Palette options control screen type:
  • T: 24 bit true-color
  • H: 16(15) bit high-color R5G6B5 (R5G5B5)
  • 8: 8 bit palette based (256 color)
  If no palette is specified, or a selection fails, will fallback from 24 to 16 to 15 to 8 then fail. If the ENV variable 'POVRENDERSCREEN' is set, it's contents will be used as the name of a public screen to open a Picasso96 PIP window on. If the screen is not found, it will fallback to the default pubscreen.
• 'C' CyberGfx support. Same palette options as Picasso96. Does not currently support windows, just full screen.

Auto-Interlacing has been removed, when updating to use BestModeID() to choose screen resolution. There is still no way to force interlacing, so if you (for example) render a 200x200 image, it will always (probably...) be non-interlaced lo-res.