Decoder hwaccel flag is too simplified

[fda0]

Recent commit 5504d7b simplifies how hwaccel flag works. Now it is a boolean. Unfortunately this leaves less control for the end user and can lead to problems.

For example we target CUDA in our code and if we set environment variables like: DISPLAY=:0 it will assume 'vdpau' as our hardware target.

Having a possibility to specify 'cuda' manually would be less error prone for us.
Simply reverting that commit would fix our issue.