message: fix(text): fix gradient text background cause render error
diff: import { TextAlign, TextVerticalAlign, ImageLike, Dictionary, MapToType } from '../core/types';
  import { parseRichText, parsePlainText } from './helper/parseText';
  import TSpan, { TSpanStyleProps } from './TSpan';
  -import { retrieve2, isString, each, normalizeCssArray, trim, retrieve3, extend, keys, defaults } from '../core/util';
  +import { retrieve2, each, normalizeCssArray, trim, retrieve3, extend, keys, defaults } from '../core/util';
  import { DEFAULT_FONT, adjustTextX, adjustTextY } from '../contain/text';
  import ZRImage from './Image';
  import Rect from './shape/Rect';
  @@ -803,13 +803,14 @@ class ZRText extends Displayable<TextProps> {
  const textBackgroundColor = style.backgroundColor;
  const textBorderWidth = style.borderWidth;
  const textBorderColor = style.borderColor;
  - const isPlainBg = isString(textBackgroundColor);
  + const isImageBg = textBackgroundColor && (textBackgroundColor as {image: ImageLike}).image;
  + const isPlainOrGradientBg = textBackgroundColor && !isImageBg;
  const textBorderRadius = style.borderRadius;
  const self = this;
  let rectEl: Rect;
  let imgEl: ZRImage;
  - if (isPlainBg || (textBorderWidth && textBorderColor)) {
  + if (isPlainOrGradientBg || (textBorderWidth && textBorderColor)) {
  // Background is color
  rectEl = this._getOrCreateChild(Rect);
  rectEl.useStyle(rectEl.createStyle()); // Create an empty style.
  @@ -823,12 +824,12 @@ class ZRText extends Displayable<TextProps> {
  rectEl.dirtyShape();
  }
  - if (isPlainBg) {
  + if (isPlainOrGradientBg) {
  const rectStyle = rectEl.style;
  rectStyle.fill = textBackgroundColor as string || null;
  rectStyle.fillOpacity = retrieve2(style.fillOpacity, 1);
  }
  - else if (textBackgroundColor && (textBackgroundColor as {image: ImageLike}).image) {
  + else if (isImageBg) {
  imgEl = this._getOrCreateChild(ZRImage);
  imgEl.onload = function () {
  // Refresh and relayout after image loaded.
  