[Feature Request] Add a function to colorize VertexData with a gradient

[Fancy2209]

I'm getting some more stuff implemented into Canvas (since I assume that bug that was fixed a few commits ago could warrant a Starling 2.8.1, I'm making sure to get some more improvements in :D) and for Gradient Fills it would probably be ideal to use Vertex Colors instead of a bitmap.
For this reason I suggest adding something like VertexData.gradientColorize() that takes arguments similar to Graphics.beginGradientFill() that colorizes Vertex Data with a gradient instead of a uniform color