vpx_alpha: huge memory leak when playing VP9 video

Source: any vp9 video without alpha.

Result:

changed milestone to %4.0

added Component::Decoders label

assigned to @alexandre-janniaux

Attached memory graph.

vlc_02.memgraph

mentioned in merge request !6018 (merged)

closed with merge request !6018 (merged)

closed with commit a3054e86