Add auto-detection and usage of GoG installations of RTCW (similar to Steam)

[JoelTroch]

Hello,

Right now, RealRTCW is able to detect and use a Steam installation of Return to Castle Wolfenstein if it finds one. It would be nice to have the same feature for the GoG version. To be perfectly clear: when I say "GoG version", I'm talking about the offline installers that you download from GoG's website, not the launcher: GoG Galaxy.

ioRTCW already implement this feature and here are various links to said implementation in the SP code branch that could be used as reference (I might have missed some, sorry if that's the case):

• GoG ID definition of RTCW: https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/qcommon/q_shared.h#L70
• char *Sys_GogPath() declaration: https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/qcommon/qcommon.h#L1170
• fs_gogpath CVAR: https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/qcommon/files.c#L286
• Search files in fs_gogpath when opening them: https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/qcommon/files.c#L909-L922
• PK3 lookup: https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/qcommon/files.c#L2876-L2883
• Mounting GoG path: https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/qcommon/files.c#L3608-L3611
• Ditto but based on fs_basepath: https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/qcommon/files.c#L3639-L3641
• Ditto for fs_game: https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/qcommon/files.c#L3660-L3662
• Unix "stub" implementation of char *Sys_GogPath(): https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/sys/sys_unix.c#L148-L157
• Win32 implementation of char *Sys_GogPath(): https://github.com/iortcw/iortcw/blob/438e7d413b5f7277187c35b032eb0ef9093ae778/SP/code/sys/sys_win32.c#L200-L231

Thank you