Provide an option to return base64-encoded images in CI log

Some CI services don't have good support for artifacts. In those cases we could print out to the log the base64 expected and actual image, inside a script to generate these files. Then one can simply copy/paste the script, run it locally, and get the images. See https://github.com/WorldWideTelescope/pywwt/blob/master/pywwt/tests/test_qt_widget.py#L67 for an example of this.

This could be triggered e.g. by an environment variable or a command-line flag (--mpl-base64).

Opened this following mention of this problem in https://github.com/matplotlib/pytest-mpl/issues/81

首先阅读 pywwt/tests/test_qt_widget.py 中第 67 行附近引用的示例，然后检查 pytest-mpl 现有的命令行和图像比较入口点。实现一个 opt-in 环境变量或 --mpl-base64 模式，使其输出包含预期图像和实际图像的脚本，以用于 CI 日志，并验证生成的脚本能够在本地重新创建这两张图像。