Add scroll speed factor info for changing scroll speed multiplier in map

[WilliamQiufeng]

Scroll speed factor changes can introduce parallax effects and help making some SV effects more smooth.

Current implementation consists of a list of factors that are lerped in time (excluding start and end). Each factor info has a LaneMask field, that is a bitwise mask of the lanes (0-indexed) that are affected by this factor, ith bit for the ith lane: -1 applies to all lanes, -56 (1111...1001000) for the 4th and 7th lane in a 7k map (1-indexed). This way we are able to have separate factor changes per lane.