Pixel Art Upscaling Shader
GMS2 Port of Casey Muratori's pixel art upscaling shader, designed to scale pixel art to awkward resolutions using non-integer scaling factors. The original shadertoy source can be found here and further described in this video. More pixel art filtering shaders/functions can be found here (it's a really good page, give it a look).
I can't find a license for the shader code anywhere so we have to presume Casey's work is licensed as All Rights Reserved. If you'd like to use this code in your game then you'll need to ask him for an appropriate license. The GMS2 port portion of this project is MIT Licensed, for whatever that's worth.