v0.3.0

• Rename BlendMode::Lrgb -> BlendMode::LinearRgb
• Add BlendMode::Oklab
• Support for smoothness in hard-edged gradient.
• Rename lots of preset gradients to snake case:
  • brbg -> br_bg
  • piyg -> pi_yg
  • and others.
• Using uniform B-splines to interpolate preset gradients.
• Some bug fixes.