Fix for screenshots taken in OpenGL with resolution 1366x768

Fix for this bug: https://mb.srb2.org/showthread.php?t=39882 (also reported here: https://mb.srb2.org/showthread.php?p=793077#10)

This is basically porting GZDoom's own fix for the same issue (I stumbled on a Zandronum thread about the same issue from google images, and one user mentioned GZDoom had fixed it): https://github.com/doomtech/gzdoom/commit/d31a0b77fd19dae611166265acabe666c7ab1c59