When swapping colors a fully transparent color (alpha=0) is changed to be fully opaque (alpha=255)

[LiquidFun]

Alright, it's me again :D

Pixelorama version: v0.8.2-dev, Commit "Fixes #363" (df7a650) (latest)

OS/device including version: Arch Linux

Issue description:

If one of the colors is fully transparent when swapping between the left-tool color and right-tool color it will become fully opaque.

Steps to reproduce:

Click on one color and drag the alpha slider to 0, then press x or on the two-sided arrows to swap the colors.

I see why it happens here:

Pixelorama/src/Autoload/Tools.gd

Lines 122 to 129 in df7a650

	func assign_color(color : Color, button : int) -> void:
	var c : Color = _slots[button].color
	if color.a == 0:
	if color.r != c.r or color.g != c.g or color.b != c.b:
	color.a = 1
	_slots[button].color = color
	Global.config_cache.set_value(_slots[button].kname, "color", color)
	emit_signal("color_changed", color, button)

But I can't say I understand the reasoning behind that if statement? At least the swapping behaviour is not what one might expect from other software.

[OverloadedOrama]

I think that this behavior comes from suggestion #54, but I did not realize that this piece of code also affects swapping. I will fix this so swapping will work as intended.