True color

[tznind]

Hey, I've done a bit of work on true color. Changes include

• Merging TrueColorAttribute into Attribute
• Writing a mapping from RBG to ConsoleColor using HSV nearest
• Added an Image senario
• Refactor tests to more easily add text cases and cover +/- 5 when

One thing is that if true color is enabled in Driver then the true color RGB values are always used. This means that if different OS/console map the console colors to different RGB that difference will not be apparent. Worth discussing further anyway.

I have merged develop into my branch so you should see an easier to read diff if you sync your branch.

I think some more work can be done on the mapping. I did start with just

distance = (R1-R2)^2 + (G1-G2)^2 + (B1-B2)^2

But this seemed to be a bit meh so I experimented with HSL distance instead. But Hue for black/grey/white are undefined which makes it a bigger pain.

The current algorithm pushes anything that's not 100% black to a color so I will have to tinker and try to fix that.

The PR adds a dependency to UICatalog only (not the main library) on ImageSharp which is the de-facto standard imaging library for .net (as well as the one that Spectre.Console are wrapping for imaging work).

[tznind]

Update:

Ok I think HSV for mapping is not working out so well and RGB just works better. have removed it in
bb81bc9

For reference here is a comparison:

[tznind]

@veeman are you still interested in working on true color support and trying to get your feature branch true-color-support into Terminal.Gui?

If so then please let me know how you feel about the changes in this PR to that feature.

If not then I can retarget this at the main Terminal.Gui repo as a new PR and work on it there.