Tile prefetch creates sometimes invalid png files
As mentioned here: https://github.com/MapWindow/MapWinGIS/issues/117
Sometimes 363 bytes PNG-files are saved, which are invalid.
Because they are saved MapWinGIS sees the files and skips getting new ones.
I created a unit test and sometime GoogleHybrid throws
Most likely this is the place when the invalid PNG-file is created.
I will investigate this more.