Add ffmpeg nlmeans_opencl support

[E-t-z]

Nlmeans is great filter to reduce noise on very grainy old sources, but it is painfully slow.
ffmpeg also supports opencl accelerated nlmeans_opencl, which is way faster as it uses GPU for denoising.
Unfortunately it is not possible to use it in Fastflix at the moment, only software nlmeans is available.

[E-t-z]

Unfortunately had not time to look into this, yet.

[cdgriffith]

Just an update. In 4.7.0 I am now checking for OpenCL support on launch, and can init the hardware device. so closer to making this a reality, but need better filter control / how to hwupload / hwdownload when needed with the filters.