Added VertexBufferUpdateCallback

Added VertexBufferUpdateCallback

Added the callback invoked when the vertex buffer needs to be updated.
The callback passes pointer and maximum size that should be written.

The callback returns number of valid elements to draw.

In case there are more than 1 vertex buffers attached, the render geometry iterates and looks for lowest number of elements to render (so we won't go out of boundaries of any of attached buffers).

The update of VertexBuffer through the callback takes place on the update/render thread and it's up to the user to ensure explicit synchronization. Event side DALi objects must not be accessed (most likely, that will fail) so alternative way of
passing data should be added by developers.

Change-Id: I4db7812cf6d6579c8cd05bcc5c25d966f6ae1aad