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.

先定位 Plotly.NET.Export 和現有的圖像匯出測試。了解匯出的 base64 圖像如何解碼，然後確定如何使用平均偏差閾值逐像素比較參考圖像。完成的標準是圖像匯出測試接受等效編碼，同時偵測出有意義的視覺差異。