Resizing a RenderTexture for PixelPerfectCam

Im relatively experienced in shader programming and Im wondering how I would go about resizing the Camera Image first to 320x180 and then that back to 1920x1080. I dont want the lame "just put a RawImage with an downscaled rendertexture in your canvas" because that solution is bs. Also it would be great if I could do it in one shader with as little code in c# as possible. I guess my question could also be: How would I go about creating a pixel perfect shader. Thanks in advance... (PS: I want to make my own one, cause I would like to be able to edit it)