Unstable screenwipe algorithm

The crossfade screen melt replacement in Strife uses the XLATAB translucency table to calculate intermediate screens between the beginning and end screen, and continues to do this until it has detected that no pixel on the entire screen has changed.

During Chocolate Strife development it was discovered that the algorithm's implementation is highly unstable. It requires both a perfectly constructed XLATAB lump for the game palette and the precise behavior of the Doom engine's screen buffer refresh mechanism in order to work without modification. Otherwise, the code will enter an infinite loop as the steady state it expects to eventually reach will never occur. As a result, the code had to be modified slightly to use a temporary screen buffer not required in the original game engine, to ensure that the contents of the framebuffer would not change unpredictably during the wipe process.