Add CacheMode option for GDALImageReaderWrapper with default option Cache_By_Line

In most cases, the pixels are accessed sequentially by line. However, the cache was realised by square. Hence, requested pixels are very frequently missing in cache -> enormous delay.

Solution: Add CacheMode option for GDALImageReaderWrapper with default option Cache_By_Line. In particular cases like pyramid viewers, we can make it cache by square.