Standardize the behavior of FF_INVERTSIDES and FF_INVERTPLANES

This is what FF_ALLSIDES, FF_BOTHPLANES, FF_INVERTSIDES and FF_INVERTPLANES claim to do on an FOF, according to their own names, the comments in the source code and the description on the wiki:

• FF_INVERTSIDES/FF_INVERTPLANES: Don't render outsides, but render insides
• FF_ALLSIDES/FF_BOTHPLANES: Render both insides and outsides
• FF_ALLSIDES|FF_INVERTSIDES/FF_BOTHPLANES|FF_INVERTPLANES: Render both insides and outsides

This is what they actually do (for the most part):

• FF_INVERTSIDES/FF_INVERTPLANES: Don't render outsides
• FF_ALLSIDES/FF_BOTHPLANES: Render insides
• FF_ALLSIDES|FF_INVERTSIDES/FF_BOTHPLANES|FF_INVERTPLANES: Render insides, but not outsides

And then there are some parts of the code where they do a weird hybrid of the two. This branch makes them do what they're supposed to do.

Test files:

• srb2win-invert.exe
• invert.wad