Vic 20 dithering support added (preliminary) [post refactor]

[robin-raymond]

The Vic 20 has some unique attributes that are similar but different than the Commodore 64. The Vic 20 doesn't have a true graphics mode but instead relies on character re-imaging and color choices to make a pseudo bitmap. Combined with the Vic 20's flexible screen sizes, this makes an interesting target.

Some key differences added:

• In multi mode, the Vic 20 has the choice of 4 color options just like the C64, except three of the choices are the background, border, and auxiliary color. These are global colors, and then each block gets the choice of its own foreground color.
• In hires mode, the choice is the background or a foreground color for the block
• In multi-mode the palette is fully available for the background, and aux color but a palette subset is used for the aux color and the foreground color.
• In hires mode, the palette is restricted for the background and the foreground color.

Code changes (some refactoring):

• The global color selection is improved, because the choice of global colors is so critical for the vic, the background, aux, and border colors are selected by pixels, whereas commodore 64 only uses the background (at times), the logic now first uses histogram usage to pick the top N colors based on usage if the color is useable by the image's pixels. The remaining colors that are needed to have compatible colors are based on the intensity and are selected for palette compatibility to the image rather than overall usage.
• the palette options got a lot more complex in the settings so those got split out into their own dithering settings with appropriate defaults that would mean the rest of the settings don't need to specify anything and they will still work
• the pixel color choices now respect the palette subset that can happen in the case of the Vic 20
• added VIC 20 palette colors (there are some differences between systems but this is true of the C64 as well)
• added export bin routines for the Vic 20 which are likely to produce nothing much of useful value given they aren't tested against working code
• misc typos and a few inconsistencies in the code (code could use a good auto-format tool as I find the editors liking to do it the way they think the code should be not how the code is)

This replaces PR: https://github.com/sehugg/dithertron/pull/19

[robin-raymond]

Related to issue https://github.com/sehugg/dithertron/issues/18

[sehugg]

Looks great! I didn't know the VIC-20 had such capabilities, it doesn't look half bad. I'm trying out a new system select menu, it hides the sub-options until you select the main one. I think it's a little more palatable for casual users but still a WIP. Thanks again!