Add opacity control to video texture streaming

Modified fragment shader of StreamVideoMaterialShader class
to support alpha. Added m_id_opacity member to the class
and update it with state.opacity.

Change-Id: I92173fac84c48862fb92f3a6338acf8c9bf91bc1
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>