// spherical billboard: per-particle view-facing basis, so no camera right/up uniforms are needed.
// requires _graphics_uniforms.glsl
vec3 billboardWorld(vec3 center, vec2 quadPos, vec2 cosSin, float size) {
    vec3 viewDir = normalize(u_cameraPos - center);
    vec3 right = cross(vec3(0.0, 1.0, 0.0), viewDir);
    float rlen = length(right);
    right = (rlen > 0.001) ? (right / rlen) : vec3(1.0, 0.0, 0.0);   // degenerate: looking straight up/down
    vec3 up = cross(viewDir, right);
    vec2 corner = vec2(
        (quadPos.x * cosSin.x) - (quadPos.y * cosSin.y),
        (quadPos.x * cosSin.y) + (quadPos.y * cosSin.x)
    );
    return center + ((right * corner.x) + (up * corner.y)) * size;
}