encodebin/decodebin: va/non-va does not play happily together as good children should

Submitted by Marc Leeman @den_erpel

Link to original bug (#748686)

Description

The problem is likely in essence similar for encoding and decoding.

When using CPUs that have some form of VA (e.g. QuickSync) and when the gst-vaapi elements are installed, when encoding and/or decoding; the va variant of the decoder/encoder is chosen (software and hardware elements seem to have the same rank though).

This is fine in most cases where a user wants to decode a single video.

When more streams are processed, it is easy to overload the system (as seen in framedrops for encoding and/or decoding), even thought the system has capacity left.

On top of that, it is seen that, when the vaapi elements are installed on a non-VA enabled system; it will break the functionality (tested for encoding at least).

Improvements would be:

1/ encodebin (and decodebin) only plug in the va variants on supported CPUs. Maybe the va elements should return when there is no support.

2/ It should be configurable to use the va/non-va variant on a higher level so that e.g. the first 6 instances use the va variants and others the non-va variant. Would it be a good idea in a mechanism like playbin where the diplay element can be specified as a property?