Handle alpha_zero flag

[Artyom Pavlov]

-K option forces FLIF codec to keep RGB values for pixels with alpha=0. If we'll encode RGBA image (for example rust logo) without this option this crate will produce the following result:

flif CLI tool produces correct output:

Probably there is a bug in alpha_zero flag handling.

Note that this issue is different from #55, as image in it is encoded with -K option.