Android: refactor video renderer.

Removed the overhead of having to create a shared OpenGL context in the GUI
thread and pre-render the frame into a FBO.

We now directly render the GL_TEXTURE_EXTERNAL_OES in the QtQuick render
thread, using an Android-specific QSGVideoNode.
We also use a callback from the render thread to create the texture from
there and not have to create a separate shared OpenGL context.

Change-Id: I6c8eb94b47d0a03329c912701b8af3fb5ebd1876
Reviewed-by: Christian Stromme <christian.stromme@digia.com>