rdp: the alpha value that goes to alpha compare in two-cycle mode comes from the 1st alpha combiner cycle of the next pixel, is modified by the next pixel's coverage value, is unaffected by chroma keying and is dithered by the current pixel's dithering value; "shade alpha" in the 2nd blender cycle comes from the next pixel, yet it is also dithered by the current pixel's dithering value; reversed the two-cycle mode behavior of span-final pixels with respect to LOD, alpha compare and convert_one; added missing sign extensions that could previously thwart triangular filtering if convert_one was set; simplified some LOD code.
