Image comparing tests for ImageExport

There's Plotly.NET.Export which exports images, for example, into base64. However testing it on exact matches doesn't work well, after all, there are multiple ways the same image can be encoded with jpg or png.

So the idea here is to convert it to an image and compare pixel by pixel against a model one. If the deviation on average isn't big, then the test is considered to pass.

首先定位 Plotly.NET.Export 和现有的图像导出测试。了解导出的 base64 图像是如何解码的，然后确定如何使用平均偏差阈值逐像素比较参考图像。完成的标准是图像导出测试接受等效编码，同时检测出有意义的视觉差异。