[skin] Add ShadedMeshOnTrack && some background helpers

[itszn]

This PR adds a few features to make mod-charting a bit easier in USC.

• ShadedMeshOnTack - Extends ShadedMesh to allow rendering with the track's world transform. This makes it very easy to render something moving on the track itself (such as fake notes)
  • The class also holds an position and a scale vector which are applied before rendering to simplify object movement
  • mesh:UseGameMesh can be called to use built in game meshes for buttons, fxbuttons, and the track itself. This also sets the length value which is used for more convenient scaling operations with mesh:ScaleToLength

I also added a few helper methods to the lua background class:

• track.GetCurrentLaneXPos - Gets the current x (relative to the track) for a given lane. This is constant unless the track is split.
• track.GetYPosForTime - Gets the y value for a given timing point in the song. This makes it easy to animate meshes moving down the track, tied to a given time
• track.GetLengthForDuration - Given a range of time (start and duration) returns a length relative to the track. This is useful for creating fake long notes, or scaling a mesh for a certain duration
• track.HideObject - This will tell the game to not render a hit object at the given time in the given lane. This allows the background to remove notes from the track allowing them to be animated separately

Example mod chart using these features: https://vimeo.com/560297079

I still need to add documentation, and potentially a few more values which may be convenient (such as button widths)