#82 Disable automatic play of internal wad demo lumps

[Blondie]

The ability to disable automatic play of internal demo lumps in iwads and pwads would be nice. They're particularly annoying when navigating the menu without starting a new game, because the menu pauses briefly whenever a new demo is started. Also, any megawads that don't contain replacement internal demo lumps will cause a desync with the iwad demos, making them pointless.

With the internal demo lumps disabled, the titlepic lump and the prboom-plus credits screen could be displayed back and forth, similar to ZDoom's menu behavior. This would be very convenient to avoid having to start a new game every time you want to navigate the menu immediately after launching the exe.