Use an orthographic projection and position a pixel at the middle of a pixel ie 14.5,27.5 is the middle of the 14,27 pixel.
If you are using a 3d perspective projection you can still switch to orthographic temporarily, draw your precision stuff, and switch back to 3d. Another option is that second one mentioned, make a quad filling the screen using a texture the exact size of the screen (or texture coords that match) and then edit the texture.
If you are using a 3d perspective projection you can still switch to orthographic temporarily, draw your precision stuff, and switch back to 3d. Another option is that second one mentioned, make a quad filling the screen using a texture the exact size of the screen (or texture coords that match) and then edit the texture.