Expose srcOffset and length of bufferSubData in VertexBuffer.data

[bullno1]

Use case: I'm writing a sprite batcher which fills a CPU vertex buffer every time a new sprite is drawn.
When the buffer is "full" or when there are no more sprite to draw, it will upload everything into a VBO and render.
To make partially filled buffer uploads more efficient, it is necessary to specify the length. srcOffset is a "might as well" add since it is part of bufferSubData.