texture is generated without mipmaps, try enabling them, find line, set true: drawingTexture = new Texture2D(texWidth, texHeight, TextureFormat.RGBA32, false); and maybe this to true also, find: drawingTexture.Apply(false);
maybe better with filtermode: bilinear/trilinear (inspector)
https://www.youtube.com/watch?v=nl8zT06yn3M