Implement rate adaptation

[podborski]

So far we mix only 2 resolutions (high and low) but we could also mix different quality levels (QPs) of different tiles. Hence:

• Add rate adaptation UI controls to the player so that the user can select certain video quality (QP) for all tiles.
• Add pre-defined rate adaptation logic which can also be selected by the user, e.g.: low QP for high res tiles and high QP for low res tiles.
• Gradually increase QP based on the distance from the viewport, etc