Compress rapid-fire screenshots

[nmlgc]

The original game's screenshot feature (#15) only writes uncompressed .BMP files that take up quite a bit of disk space. Due to the rapid-fire nature of these screenshots, this can add up to hundreds of MB very quickly.

Nowadays, there are two main contenders for lossless compressed image formats:

• PNG: Widely supported, best compression, but very slow. So slow that it might cause frame drops when holding the screenshot key.
• QOI: Released in November 2021, quickly became famous for its simplicity and encoding speed, and is quickly gaining support in other programs. However, its compression algorithm was not built with heavily dithered 256-color source images in mind, and it would therefore compress Shuusou Gyoku screenshots rather poorly. Just mentioning it here because I'm sure that someone would recommend it otherwise.
• GIF: Would actually compress only slightly worse than PNG, but 256 colors are probably not enough once Direct3D-rendered transparent shapes come into play.

None of these are part of SDL, so we'd have to add another library in any case.

Here's a decently busy reference screenshot:

Format	File size	Encoding speed
BMP, 32-bit	1,228,854 bytes	🏎️
QOI	179,201 bytes	🏍️
GIF, optimized	61,290 bytes
PNG, optimized	46,681 bytes	🐌

Despite the slow compression, I'm still heavily leaning towards PNG. If the compression does result in frame drops, the resulting slowdown can provide a nice feedback to players that their hard drive is currently blasted with new image files.