Ambient shadow tessellation improvement.

Using the vertices, instead of ray casting for the triangulation.

This request a dynamic index buffer associated with vertex buffer,
so we update the VertexBuffer to support it.

The ambient shadow could be 3x-6x times faster for circle and rect now.

b/16712006
b/14257173

Change-Id: I2f22a8fe19bc59acee5c18e4a3a395acd5042a66