Find better color-to-object mapping for GLSL picking

At the moment the Picking Module computes pick shader colour for each object (node) thus:

  color.g = parseFloat(Math.round((nodeIndex + 1) / 256) / 256);
  color.r = parseFloat((nodeIndex - color.g * 256 + 1) / 256);
  color.b = 1.0;

For each object (node) rendered, that colour is loaded into a singleton picking fragment shader:

uniform vec3 uPickColor;
    void main(void) {
            gl_FragColor = vec4(uPickColor.rgb, 1.0);
    }

Once frame is rendered, the colour is read from the framebuffer:

 function readPickBuffer() {
        var context = boundPickBuf.canvas.context;
        var pix = context.readPixels(pickX, boundPickBuf.canvas.canvas.height - pickY, 1, 1, context.RGBA, context.UNSIGNED_BYTE);
        if (!pix) {  //  http://asalga.wordpress.com/2010/07/14/compensating-for-webgl-readpixels-spec-changes/
            pix = new WebGLUnsignedByteArray(4);
            context.readPixels(pickX, boundPickBuf.canvas.height - pickY, 1, 1, context.RGBA, context.UNSIGNED_BYTE, pix);
        }
        if (debugCfg.logTrace) {
            SceneJS._loggingModule.info("Reading pick buffer - picked pixel(" + pickX + ", " + pickY + ") = {r:" + pix[0] + ", g:" + pix[1] + ", b:" + pix[2] + "}");
        }
        pickedNodeIndex = (pix[0] + pix[1] * 256) - 1;
    }

However the pickedNodeIndex does not always map to the original nodeIndex, probably due to inaccuracies in numeric translation between JS and GLSL.