Write a TGA file with Zig

This program demonstrates writing TGA “targa” image files by writing a pretty gradient.

TGA is easy to write and has wonderfully software support, but there are a couple “gotchas” in the header and image data, namely:

• An image “origin” is specified in two places in the header - get it wrong and you’ll be scratching your head at the mess on the screen.
• Pixels are in Blue Green Red order, not RGB.

https://en.wikipedia.org/wiki/Truevision_TGA