Consider adding mutables to styling language

[pjcozzi]

via @SunBlack:

{
    "mutables" : {
        "lengthFactor" : { type: "float", value: 0.1 },
        "idFactor" : { type: "int", value: 5 }
    },
    "defines" : {
        "length" : "length(${POSITION})",
        "time" : "${tiles3d_tileset_time} * 0.1"
    },
    "color" : {
        "conditions" : [
            ["${length} < ${lengthFactor}", "${length} * color('red')"],
            ["${length} < 5 * ${lengthFactor}", "vec4(vec3(${temperature} * fract(${time})), 1.0)"],
            ["${id} < 150 * ${idFactor}", "vec4(${secondaryColor} / 5.0, 1.0)"],
            ["${id} < 200 * ${idFactor}", "rgb(0, 0, Number(${secondaryColor}.x < 0.5) * 255)"]
        ]
    },
    "show" : "${length} < 0.9"
}

Full discussion: CesiumGS/cesium#5380 (comment)

[lilleyse]

A CesiumJS implementation was started in CesiumGS/cesium#5380. The code is parked at Cesium3DTileStyle_uniform_support.

[lilleyse]

CesiumJS has custom shaders which may be a better approach.

May also be considered for future styling language updates: #2 (comment)