When trying to add a set of vertices with AddUIVertexTriangleStream, this function replaces the vertices instead. Also, it doesn't update UV2 and UV3.

Steps to reproduce:
1) Open the attached project.
2) In the Hierarchy, select Canvas > Image.
3) On the attached script, toggle 'Use Set Vert'.
- The image in Game view becomes black.

This happens because AddUIVertexTriangleStream doesn't write to UV2/UV3. The following call is used internally:
CanvasRenderer.SplitUIVertexStreams(verts, m_Positions, m_Colors, m_Uv0S, m_Uv1S, m_Normals, m_Tangents, m_Indices);

'GenerateUV2andUV3.cs' is responsible for the calls in this project.

In Mono backend, SplitUIVertexStreams replaces the vertices, when it should be adding them.

Reproduced on:
2017.1.0b1, 5.6.0f3, 5.5.2p4