This also adds support to avconv (which is trivial due to the new
hwaccel API being generic enough).

The new decoder setup code in dxva2.c is significantly based on work by
Steve Lhomme <robux4@gmail.com>, but with heavy changes/rewrites.
Signed-off-by: Diego Biurrun <diego@biurrun.de>

Not yet enabled for any hwaccels.

This unclutters the top-level directory and groups related files together.