keep alpha channel

[lxzmxl]

use opencv cv::imread to read a png,it's a 4 channel cv::mat with alpha channel.
but when I use libimagequant to compress that png file and write to disk, opencv read it as a 3 channel cv::mat which lost alpha channel.
how can I solve this problem.
@leahneukirchen @mdonoughe @jaalto @rflynn

[kornelski]

PNG8 files don't have a separate alpha channel, they have transparency in palette entries. This should be possible to convert to a 4-channel image. Reading these files as 3-channel looks like an error.

I've seen people recommend IMREAD_UNCHANGED flag. Have you tried this?