Post 742 (color!) followups

[waywardmonkeys]

Once #742 lands, there some stuff that we should fix up and improve and I don't want to keep expanding the scope of #742:

• Remove ColorF64: A first pass wouldn't use the good gradient interpolation yet, just use DynamicColor instead.
  This is complicated by the fact that gradients and non-gradient color data are being written in different byte orders.
• Actually do gradients.
• Use a consistent byte order for color data being sent to the GPU?
• vello_encoding::RenderConfig::new doesn't need to take a reference to the base color, nor does it have to use peniko::Color.
  Instead, it can perhaps use impl Into<DrawColor> and simplify calling code as that allows a variety of color types to be used.
  This would probably not change a lot though since we don't want to use DrawColor in RenderParams from the main vello crate.
• Switch more example code from using Color::from_* or Color::new to using colors from color::palette::css where the actual color doesn't really matter.
  Notably, some of the examples should be able to entirely stop using peniko::Color.
• pico_svg's parse_color has some fallback for rgb(...) syntax that is probably not needed anymore.
  pico_svg: Simplify color parsing. #753
• Consider changing some to_u32 methods in color to be to_be_u32 and to_le_u32 or some other naming and making it more clear up through the stack.
• DrawColor has an comment about the endianness, so when fixing things above, make sure that this is corrected.
• Why does the blurred rounded rect code only support a color as a brush and not impl Into<BrushRef<'b>>?