VAAPI: Some fixes from weekend

According to the tests I have done here:
![1080p24-1024](https://f.cloud.github.com/assets/1089309/2125049/9b7213cc-924a-11e3-9d28-909137e22d96.png)

![3840p30-1024](https://f.cloud.github.com/assets/1089309/2125050/9fb4da78-924a-11e3-8628-f6779f5e2ba7.png)

I suggest increasing the Cachesize to 8K. One can clearly see that 4K surfaces are 1.5 times faster. This is quite clear: we only get one line of a 4K surface into a 4K cache, but two with an 8K cache.

Concerning the 1080p24 one clearly sees, that the 8K cache has much less outliers and therefore is more stable.

The other one https://github.com/fritsch/xbmc/commit/3185f81923dae0ba351c1f42e661920a8c4b468c I found while reading.