videocommon: UpdateDrawRectangle cleanup

I felt this code was pretty messy so I wanted to clean it up a bit. I have tested it by changing all the graphic setting options but I am unsure if that is enough. If someone could verify I havent screwed up that would be helpful.
Instead of forcing the user to enable Stretch and WidescreenHack at the same time, it might be better to keep this line as it was and to add a condition to line 393 of " && !g_ActiveConfig.bWidescreenHack ". This way the emulator will auto-enable Stretch if the user has selected WidescreenHack.