Move the SRT Prometheus metrics into its own class, for reusability.

[nageru] / futatabi / chroma_subsample.frag

#version 450 core
in vec2 tc0, tc1;
uniform sampler2D cbcr_tex;
out float Cb, Cr;
void main() {
        vec2 result = 0.5 * (texture(cbcr_tex, tc0).rg + texture(cbcr_tex, tc1).rg);
        Cb = result.r;
        Cr = result.g;
}