计算THREE.Shape()生成的形状的uv，用shader material贴图

THREE.Shape()生成的形状会打破默认uv计算方式，需要重新计算才能正确应用shader的渐变色。

function reMapUv(geometry) {
            geometry.computeBoundingBox();

            var max = geometry.boundingBox.max,
                min = geometry.boundingBox.min;
            var offset = new THREE.Vector2(0 - min.x, 0 - min.y);
            var range = new THREE.Vector2(max.x - min.x, max.y - min.y);
            var faces = geometry.faces;

            geometry.faceVertexUvs[0] = [];

            for (var i = 0; i < faces.length ; i++) {

                var v1 = geometry.vertices[faces[i].a],
                    v2 = geometry.vertices[faces[i].b],
                    v3 = geometry.vertices[faces[i].c];

                geometry.faceVertexUvs[0].push([
                    new THREE.Vector2((v1.x + offset.x)/range.x ,(v1.y + offset.y)/range.y),
                    new THREE.Vector2((v2.x + offset.x)/range.x ,(v2.y + offset.y)/range.y),
                    new THREE.Vector2((v3.x + offset.x)/range.x ,(v3.y + offset.y)/range.y)
                ]);
            }
            geometry.uvsNeedUpdate = true;
        }