Add plusDarker and plusLighter blend modes

[mswpearson]

RNSkia supports all but two of the blend modes specified in this doc -> https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/mix-blend-mode#syntax

The missing two are plusDarker and plusLighter.

Is there any scope to add support for these? Or possibly a way to define custom blend modes?

These blend modes create a unique look that can't really be replicated by using any of the other available blend modes currently.

[wcandillon]

Hello Matthew 👋

Thank you for letting me know about these two blend modes.
Indeed these are not available by default in Skia. Right now the way to add them would be to use:

const plusDarkerShader = Skia.RuntimeEffect.Make(`
  uniform shader src;
  uniform shader dst;
  
  half4 main(float2 xy) {
    half4 s = src.eval(xy);
    half4 d = dst.eval(xy);
    
    // Plus Darker formula: max(0, src + dst - 1)
    // With proper alpha handling for premultiplied colors
    half3 result = max(half3(0), s.rgb + d.rgb - d.a);
    half alpha = s.a + d.a - s.a * d.a;
    
    return half4(result, alpha);
  }
`);

However I think I will add these two to the C++ API directly so they will be available in JS like a regular BlendMode.

[wcandillon]

I added it at #3654 and will publish it asap.