Squash images (pngcrush version)

[hugovk]

For #165 and #200.

Like #201 but with lossless pngcrush instead of lossy pngquant.

What	How big	Percent of original
master	3,230,356 bytes (3.4 MB on disk)	100%
after script	1,423,814 bytes (1.5 MB on disk)	44%

(Edit: updated after script sizes)

The -brute option doesn't save much space, but we're not time critical so might as well squeeze out the last few bytes.

Option	Time	Size
With -brute	1m30s	1,423,814 bytes (1.5 MB on disk) for 62 items
Without -brute	7s	1,431,120 bytes (1.5 MB on disk) for 62 items

(Edit: updated)

[takluyver]

How are you getting those sizes? I just checked out this branch, and I'm seeing about 1.5 MB of data in assets.

[hugovk]

Good question! I also see 1,423,814 bytes (1.5 MB on disk) now.

I must have initially tested pngcrush without resizing first. I'll update with/without -brute values too.

[hugovk]

Updated, sorry for the confusion!

[takluyver]

Thanks!

As I'm not an expert in image processing, I'm inclined to go with the lossless compression for now (this PR). We can always swap in pngquant later if we need to squeeze the size even more. Does that sound reasonable to you?

[hugovk]

You're welcome!

Sounds perfectly reasonable, it's already a good saving.

[takluyver]

OK, thanks. I'm going to rebase the branch locally to cut out all the regenerations of the images, and make a couple of other manual tweaks.

[takluyver]

Continued & merged in #203.