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 (more red dots arround the average), while being only approx 4% slower.

The other one https://github.com/fritsch/xbmc/commit/3185f81923dae0ba351c1f42e661920a8c4b468c I found while reading.
Testing Hardware: Core i5 Mobile i4200U