multimedia/ffmpeg: enable VAAPI and VDPAU by default

[jbeich]

Summary

Build VAAPI and VDPAU backends, so ports using system ffmpeg can take advantage of GPU-assisted decoding. Also, propagate defaults to consumers.

Hardware acceleration in video players tends to be more reliable than, say, in browsers. However, it's often still disabled by default or can be force-disabled if not.

This may help reduce the number of bundlers.

[bapt]

While here, please also enable V4L by default it is needed to be able to have things like qtox working outofbox

[jbeich]

rP412399 already made V4L=on by default for multimedia/ffmpeg. Elsewhere the impact on consumers is less clear but if they already depend on ffmpeg this often leads to "why not?".

[cpm]

It seems right, so go ahead.

[riggs]

Yep, no objection from my side.