Allow for pegging/flipping of Doom 64-style Sector Colors

[Dark Pulse]

This is exactly what it sounds like on the tin - and yes, it actually is used in a handful of places in the game.

Doom 64 allows for setting the sector colors to be pegged to an upper wall, a lower wall, or both. This is most notably used in the game's TITLEMAP (or Doom 64 EX's MAP33), where almost all of the interior walls of the letters have a "Peg Lower Wall Color" property, which will effectively spread the sector's spectrum of colors across the entire bottom part of the linedef. Without this, the game will stretch the color across the whole of the linedef - which, as it is some 6000 units high, will mean that the walls will be horribly inaccurately colored.

In addition to this, there is the possibility of flipping the upper linedef's Pegged Color, effectively "reversing" the pegging, for the upper wall of a linedef only. (Apparently there's never a need for a lower wall to be flipped - it could be nice to add though?)

There's some pretty informative discussion (with examples) between Nevander and Graf in a Doomworld thread for his Doom 64 TC.