These are NativeCall bindings to SDL_gpu (built with SDL2, so it's compatible to SDL2::Raw).
SDL_gpu is described by its author as
SDL_gpu, a library for making hardware-accelerated 2D graphics easy. by Jonathan Dearborn
SDL_gpu is licensed under the terms of the MIT License. See LICENSE.txt for details of the usage license granted to you for this code.
You can find SDL_gpu at https://github.com/grimfang4/sdl-gpu