GMS2 Port of a 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).
An alternate approach can be found here using the "Sharp Shimmerless" technique, and another alternative can be found here.