At higher resolutions it takes too much time to save a screenshot at the
maximum compression level. So let's allow the user to set the
compression level.
This re-uses the png_compression_level setting. The default compression
level is 1 for speed, but if the user wishes to increase the compression
level (without using an external tool) and doesn't mind if the
screenshot takes more time to save then they can increase the
compression level up to a maximum of 9 (which can take quite a while).
Fixes#1527.
Valid values for png_compression_level are from 0 (no compression) to 9
(max compression). The default is 1.
v2: Use zlib Z_BEST_SPEED (1) and Z_BEST_COMPRESSION (9) defines.
Gregory Hainaut