Pixel-perfect shader in Unity ShaderLab

There is a ShaderLab built-in value called _ScreenParams.

_ScreenParams.x is the screen width in pixels. _ScreenParams.y is the screen height in pixels.

Here's the documentation page: http://docs.unity3d.com/462/Documentation/Manual/SL-BuiltinValues.html

I don't think this is going to happen. Your rendering is tied to current selected video mode and it doesn't even have to match your physical screen size (if that is what you mean by pixel-perfect).

The closest you are going to get with this is if you render at recommended resolution for your display device and use pixel shader to shade an entire screen. This way, one 'physical pixel' is going to be roughly equal to one actual rendered pixel. Other than that, it is impossible to associate physical (that is your display's) pixels to rendered ones.

This is unless, of course, I somehow misunderstood your intentions.