Add DragValues for RGB(A) in the color picker

[IVAN-MK7]

Added some DragValue widgets in the color_picker widget as input fields for managing the RGBA values.
In case the provided values result in a not valid premultiplied alpha RGBA color, a button will appear next to the input fields, to be used to multiply the values with the alpha channel.

Closes #2716.

[emilk]

It is quite confusing how this works with pre-multiplied alpha.

I think we should keep premultiplied alpha out of the UI.

Instead, unmultiply the alpha before showing the buttons, and then special-case alpha=0 (additive).

Otherwise it is very easy to end up with things like this:

[IVAN-MK7]

I've applied the suggestions, had to work around the case when Additive blending is signaled with a negative alpha on Alpha::BlendOrAdditive.
Now the DragValues take unmultiplied RGBA values and the Alpha DragValue is hidden in case the provided Alpha is Alpha::Opaque (Alpha::Opaque is automatically sent to the fn in case of a negative alpha on Alpha::BlendOrAdditive).

Given the negative alpha thing, users still can't switch to "Additive" blending when alpha is 0, even after this PR change.

[tosti007]

Awesome, I'd really like this addition! To maybe help this further I gave it a review.

[tosti007]

Added another small nit i just realised. Also: Maybe it'd be good to support f32 drag values, since that would allow for HDR color pickers (which is my use case)?

[IVAN-MK7]

Added another small nit i just realised. Also: Maybe it'd be good to support f32 drag values, since that would allow for HDR color pickers (which is my use case)?

Setting 3 decimal values seems the best option for the f32, but the layout would need a rearrange, also adding a button to switch between u8 and f32 appears necessary, which would require to add a new field needed to store the preferred choise (maybe to ui.style.visuals.).

[imsi32]

Hello, while I was looking demos, I faced hard time to get the correct colors, so I find this pull request. Because I don't know egui yet, I couldn't help with my suggestion.

I think buttons on the top doesn't look good. Instead of putting them there replacing with rgba(1,1,1,1) text will be better idea like this image:

Secondly, even though I don't understand the additive page. When I open the additive pages, alpha area disables and all page change place. So putting alpha area in end of the page will make page less shaking. Unfortunately, color changing area needs to put upper of the alpha area to eliminate color mixing between color choosing area and alpha area.

[IVAN-MK7]

I've added the

pub color_picker_input_values_type: ColorPickerInputType,

field to style's Visuals.
Also rearranged color_picker's color sliders, and made color_text_ui display unmultiplied rgba values, this is the final result:

The resulting appearance definitely needs some polish, it would be better to put buttons and drag values into a StripBuilder, but it must be imported from the egui_extras crate. Help and advices are welcome.

[IVAN-MK7]

This is the resulting UI following the recent changes:

Needs a finishing touch for the layout, I tried to make so that the change of RGBA values and input type (U8/F32) made the least amount of popping, but could't go very far without a StripBuilder.

[Ved-s]

Would be nice seeing this implemented in. Also I'd suggest adding hex color field so it's easier to copy/paste, it could accept data in few formats #rrggbbaa/#rrggbb/#rgba/#rgb/#w/#ww (last two are grayscales), but show only #rrggbbaa