optimize png images using zopflipng

[PeterDaveHello]

Using Google's zopflipng to recompress the png images losslessly.

 dev/theme-kit/bg-input-focus.png                  | Bin 143 -> 73 bytes
 dev/theme-kit/bg-input.png                        | Bin 143 -> 73 bytes
 dev/theme-kit/identification.png                  | Bin 19461 -> 14752 bytes
 dist/images/agent/bg-input-agent.png              | Bin 181 -> 89 bytes
 dist/images/agent/bg-input-focus-agent.png        | Bin 186 -> 93 bytes
 dist/images/agent/sprite-agent.png                | Bin 16545 -> 8809 bytes
 dist/images/aristo/bg-input-aristo.png            | Bin 192 -> 113 bytes
 dist/images/aristo/bg-input-focus-aristo.png      | Bin 186 -> 106 bytes
 dist/images/aristo/sprite-aristo.png              | Bin 20542 -> 18749 bytes
 dist/images/default/bg-input-focus.png            | Bin 143 -> 73 bytes
 dist/images/default/bg-input.png                  | Bin 143 -> 73 bytes
 dist/images/default/sprite.png                    | Bin 34770 -> 18810 bytes
 dist/images/jeans/bg-input-focus-jeans.png        | Bin 5690 -> 4011 bytes
 dist/images/jeans/bg-input-focus-retina-jeans.png | Bin 20340 -> 8516 bytes
 dist/images/jeans/bg-input-jeans.png              | Bin 5451 -> 3881 bytes
 dist/images/jeans/bg-input-retina-jeans.png       | Bin 19484 -> 8495 bytes
 dist/images/jeans/sprite-jeans.png                | Bin 599848 -> 469896 bytes
 dist/images/jeans/sprite-retina-jeans.png         | Bin 2071761 -> 1427129 bytes

[shehi]

I had to revert this PR as it damaged PNG files by converting them from 32bit to 16bit color palette and changing them considerably in color-basis.

[PeterDaveHello]

Do all the images have the same problem? I'll be happy to look into the issue.

[shehi]

@PeterDaveHello , yes, I am afraid so, as I noticed color brightness/shade change as well as bitmap-depth (32 to 16). Please check at your end as well and let me know. Please pay special attention to sprites as they contain almost everything.

[PeterDaveHello]

@shehi would you please let me know the procedure you are using to verify the color brightness/shade change? Thanks.

[shehi]

Good old Photoshop.

[PeterDaveHello]

@shehi would you mind to share something like screenshot about the diff? Thanks.

[shehi]

[PeterDaveHello]

@shehi I use this command but found no difference:

compare -metric RMSE -verbose a.png b.png NULL:

The only difference may come from this zopflipng parameter, but it's not a visual difference:

--lossy_transparent: remove colors behind alpha channel 0. No visual difference, removes hidden information.

There are some others command can help valid the pixels, would you like to try? You know that sometimes visual make fake our eyes. BTW, actually, I use 500% size on the browser to help me found the difference, I found nothing here.