You'll need to connect an external display to be sure here whats happening. If the external image looks clear you have a problem within the internal displays logic. If the image look the same then the GPU logic is having issues.
I'm not sure your issue is the internal display its self, but the connection on the logic board. To me it looks like a signal leakage issue as you are clearly getting a herringbone pattern thought the image as well.
I'm suspecting corrosion damage here. Did you get your system wet at some point?
As you noted there was liquid damage is it near the Thunderbolt chip or the ribbon connector?
Review this image you'll see the Thunderbolt chip is green outlined on the far Right: MacBook Pro Retina - logic board (top side) You may need to pull the logic board to look at the other side: MacBook Pro Retina - logic board (bottom side) the area of concern is the top left here.
The issue is the displays data signaling between the Thunderbolt chip and the internal display its self (pathway) not the backlight circuit. But, the backlights power lines could be the source of the leakage signal into the displays data signal as both go though the same ribbon connection pathway.
I would start off cleaning off the corrosion staining as the salts within the material is conductive enough to distort the signal. I would also make an effort to clean the internal displays ribbon and the ribbon connectors contact surfaces as often I see this is where the displays signal is being altered.
Its possible the ribbon cable is damaged here which would require replacing the display assembly as trying to replace the ribbon cable alone is just not practical in this series.