Add the SKRuntimeEffect API

[mattleibow]

Description of Change

This was originally merged in a branch that was going to be m84, but the API was going to change so I delayed the release.

Bugs Fixed

• Fixes [QUESTION] SkRuntimeEffect and making the image background transparent. #1319

API Changes

SKCanvas canvas = ...; // GPU canvas

// input values
float threshold = 1.05f;
float exponent = 1.5f;

// image source
using var blueShirt = SKImage.FromEncodedData(Path.Combine(PathToImages, "blue-shirt.jpg"));
using var textureShader = blueShirt.ToShader();

// shader
var src = @"
uniform shader color_map;
uniform float scale;
uniform half exp;
uniform float3 in_colors0;
half4 main(float2 p) {
    half4 texColor = sample(color_map, p);
    if (length(abs(in_colors0 - pow(texColor.rgb, half3(exp)))) < scale)
        discard;
    return texColor;
}";
using var effect = SKRuntimeEffect.Create(src, out var errorText);

// input values
var uniforms = new SKRuntimeEffectUniforms(effect)
{
	["scale"] = threshold,
	["exp"] = exponent,
	["in_colors0"] = new[] { 1f, 1f, 1f },
};

// shader values
var children = new SKRuntimeEffectChildren(effect)
{
	["color_map"] = textureShader
};

// create actual shader
using var shader = effect.ToShader(true, uniforms, children);

// draw as normal
canvas.Clear(SKColors.Black);
using var paint = new SKPaint { Shader = shader };
canvas.DrawRect(SKRect.Create(400, 400), paint);

Behavioral Changes

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Updated documentation