Ditching RGB alpha for Kubelka-Munk optical mixing

Standard RGB alpha blending completely ruins the luminosity of digital watercolor. Layering yellow over indigo usually produces a muddy, desaturated grey in traditional digital canvases. To fix this, I wrote a 140-line render_km.wgsl shader implementing a robust 2-flux Kubelka-Munk radiative transfer model. By using true physical spectral values (absorption K and scattering S), the pigments now interact subtractively just like real minerals—yielding deep, authentic celadon greens and vibrant glazes that maintain the optical reality of actual paint.