kas::text::format::Color: use internal 32-bit RGBA value

[dhardy]

Previously, a palette index was used for text colours. This stores the colour in effect tokens instead.

Advantages:

• Easier to map to from other APIs, e.g. highlighters which don't present colours via a palette
• Saves passing palette: &[Rgba] into text drawing functions

Disadvantages:

• Limited to 32-bit (sRGB) colours
• Larger effect tokens (8 bytes instead of 4)

As a space optimisation, completely transparent text will resolve to the theme's text colour instead.