This fixes PCX loading in the non-editor build. PCX loading broke in
3114874713 because it switched from PHYSFSX_openReadBuffered to a call
which did not handle the presence of the marker byte in the filename.
The from_hog_only flag has not been properly supported since the
introduction of PHYSFSX_openReadBuffered in
8b323e7214. Editor builds ignored it on
purpose, and non-editor builds pretended to respect it, only to then
ignore it down inside PHYSFSX_openReadBuffered. For consistency with
code paths that do not use PHYSFSX_openReadBuffered, disable the bypass
in PHYSFSX_openReadBuffered and remove the code in show_title_screen
that applied the flag.
Reported-by: Jayman2000 <https://github.com/dxx-rebirth/dxx-rebirth/issues/510>
Fixes: 3114874713 ("Delegate PCX loading to SDL_image")