Fix UnimplementedError() by using double instead of int as fallback

[felixwoestmann]

This PR makes translate use double instead of int as fallback values. The reason being that when the fallback is used, the underlying function will throw an UnimplementedError(). This can happen when executing WidgetTests.

  void translate(dynamic x, [double y = 0.0, double z = 0.0]) {
    double tx;
    double ty;
    double tz;
    final tw = x is Vector4 ? x.w : 1.0;
    if (x is Vector3) {
      tx = x.x;
      ty = x.y;
      tz = x.z;
    } else if (x is Vector4) {
      tx = x.x;
      ty = x.y;
      tz = x.z;
    } else if (x is double) {
      tx = x;
      ty = y;
      tz = z;
    } else {
      throw UnimplementedError();
    }

[t-unit]

@sameersingh2704 any chance we can get this merged and published on pub.dev?