Implement possible size optimizations from Bit Depth from TruePNG

[shssoichiro]

https://css-ig.net/articles/truepng.php

The bitdepth should match to the colortype, with some exceptions in paletted mode. encoders have the choice to use 8, 4, 2 or 1 bit by pixel. most of the time, the best choice is to select the lower bitdepth, that should decrease the filesize. this feature is included in most of optimizers.

most of encoders, including TruePNG, choose the minimal bitdepth possible considering the number of colors in the image. sometimes, it is better to choose the 8 bits / pixels, even for an image that have 16 colors or less. TruePNG can check that, and it reduces filesize that way.

• Test 8 bits/pixel even if image could support less