Huh? That library exists, it is called Skia, it is the most premier 2D GPU (among other targets) rendering library in the world and it powers the Android UI, Chrome, Firefox and a ton of other things. See also: https://skia.org/
Skia is not GPU first. It was initially designed for CPU-only rendering, 3D GPU support was an afterthought. For that reason, the quality of that support ain’t great. AFAIK it only uses 3D GPU to compose layers.
No, I'm pretty sure it rasterizes using OpenGL ES, too.
I don't know the code well enough to find good proof, but there's a lot of GLES and I think some DX calls in there. More than if it was just composing.
