Convergence Offsets Don't Produce the Same Results

[akgunter]

Now that the core scanline logic is working as intended, I can see that the convergence offsets are not. I'd like them to behave more similarly to the RetroArch version of the shader.

This will be trickier. The vertical offsets are applied smack in the middle of the scanline sampling logic.

[akgunter]

This was a combination of several things that ended up requiring a near-complete rewrite of the vertical sampling logic for scanlines. The fix is in the feature/scanline-rewrite branch.