#10 lossy PNG

[Daniel Mealha Cabrita]

Add support for lossy PNG compression.
Mostly useful when dealing with files untouched by the
AllowModify=no, more specifically the transparent GIFs.

[Daniel Mealha Cabrita]

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Maybe a more viable solution would be color reduction,
allowing the user to specify max of colors allowed for that.

Also, it's appropriate to add a switch option to enable this
or not if jpeg does is not fit for recompression (because of
transparency of animated gifs).
Allowing this as an alternative to JPEG (when it is possible
to use that) is probably pointless, considering the smaller
size compression and lower quality for that size compared to
JPEG.

[Daniel Mealha Cabrita]

Files of such kind are usually animated GIFs and ones with transparency.

GIFs with transparency are usually small anyway, some optimization may occur to the bigger ones once converted to PNG.
Animated GIFs are rarer nowadays and usually are advertisement. That can be filtered by other means.

Lossy PNG compression seems too much trouble for such cases.
Lowering the colors may be an option, but may be not worth the trouble.

Lowering priority for this task.