Improve dithering of alpha gradients

// Dithering must be done in the quantization space

// When we are writing to an sRGB framebuffer, we must do the following:

//     EOTF(OETF(color) + dither)

// The dithering pattern is generated with a triangle noise generator in the range [-0.0,1.0]

// The dithering pattern is generated with a triangle noise generator in the range [-1.0,1.0]

// TODO: Handle linear fp16 render targets

const char* gFS_Gradient_Functions = R"__SHADER__(

        float triangleNoise(const highp vec2 n) {

const char* gFS_Gradient_Preamble[2] = {

        // Linear framebuffer

        "\nvec4 dither(const vec4 color) {\n"

        "    return vec4(color.rgb + (triangleNoise(gl_FragCoord.xy * screenSize.xy) / 255.0), color.a);\n"

        "    return color + (triangleNoise(gl_FragCoord.xy * screenSize.xy) / 255.0);\n"

        "}\n"

        "\nvec4 gammaMix(const vec4 a, const vec4 b, float v) {\n"

        "    vec4 c = mix(a, b, v);\n"