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>
Android: fixed image capture preview.

We used to generate the capture preview from the video output, grabbing
the pixels of the last frame (in a FBO). This is not possible anymore,
we instead query the camera for a preview frame, which is in the NV21
format and needs to be converted to RGB.

Change-Id: I1c728b3a708a6f052a83aebf9f15f511eab7a02f
Reviewed-by: Christian Stromme <christian.stromme@digia.com>