Allow transparent parts in translucent FOF and PolyObject planes
Until now, transparency in FOF flats (via the cyan = invisible hack) was only possible if you gave the FOF FF_TRANSLUCENT but set the alpha value to 256. This meant that actually translucent FOFs couldn't have transparency. The situation for PolyObjects was similar. This was despite the fact that a rendering routine that allows for both transparency and translucency does exist. This branch adds the ability to enable cyan pixel cutting explicitly via the ML_EFFECT6 flag, making translucent flats with transparency possible. To ensure backwards compatibility, the old methods for enabling transparency still work.