This reverts commit 5d36e382c7.
It's been found to cause all kinds of terrain glitches on Windows and
Linux. Please do not try experimental image compression alternatives
without discussing and reviewing first.
Overall statistics (only for files with a smaller recompressed size):
Original size: 9070 KiB on 449 files
Optimized size: 8184 KiB
Total saving: 885 KiB = 9% decrease
