Added pre-render callback to Adaptor

Added pre-render callback to Adaptor

New API to add a pre-render hook - this callback gets called from
the Update/Render thread before rendering starts. If the callback
returns false, then it removes the callback, otherwise the callback
will be called every frame.

No DALi Event API can be called from this callback, doing so will
result in instability and crashes.

To use the callback, your application needs to call the following code:

  Dali::Adaptor::Get().SetPreRenderCallback( Dali::MakeCallback( this, &SomeClass::OnPreRender ) );

or some other variant of MakeCallback.

Change-Id: I014842f0adcb004b94435353fba869d886471b93
Signed-off-by: David Steele <david.steele@samsung.com>