[imx] Deinterlacing rework

This is the rework of the HW deinterlacing for IMX6 boards (see xbmc-imx6/xbmc#70). It creates a mixer threads that offloads deinterlacing to the IPU in parallel to VPU decoding and GPU rendering.

What works:
- Selectable deinterlacing modes: None, Auto, Force

What does not work:
- Double rate feature. Can be easily implemented but needs proper settings in the GUI
- Smooth playback for HD streams which needs to be tested. The performance for my test setup increased already compared to Gotham

This PR is for review to be integrated into the current IMX6 Codec implementation.