Embed opaque images as JPEG

[illwieckz]

On #166 the PNG were optimized to save file space:

• Heavily brute-forced optimized PNG files, to save on file space when embedding them #166

But we don't need to embed opaque images as PNG, using JPEG would save more file space!

So images like unvanquished, tyrant and logo are still kept as PNG since have have transparency, but background, header and splash are now JPEG.

The PNG files of them are kept because they are the source, but they aren't embedded.

The size of those PNGs was 480K, converted to JPEG the size is 72K, so this saves 408K.

I compressed this way:

guetzli --quality 90 splash.png splash.jpg

So I used a quality of 90. If it's considered to low, the quality 95 would produce 96K for the three of them. We may also mix, the splash needs to have good enough quality, the others less.

[slipher]

The build is broken due to unwanted submodule changes.

[illwieckz]

The build is broken due to unwanted submodule changes.

Fixed.

[illwieckz]

I applied per-image quality: the splash uses quality 93, the smallest quality retaining the tyrant head scales details, and other images which aren't very detailed use quality 84 (the smallest guetzli can produce).

[illwieckz]

Before:

187867	background.png
71622	header.png
227847	splash.png
487336	total
480K	total

After:

17648	background.jpg
13057	header.jpg
37543	splash.jpg
68248	total
76K	total

[slipher]

LGTM, thanks!