debian: haliumqsg & OMX enablement patch

Part of the performance improvement story.

• Enables VP8 & VP9 video decoding using Halium's OMX services. Uses libmedia_compat_layer which 1) we already have, 2) is enough for basic decoding functionality. Could be changed in the future but the barebones Mailbox Mojo-based Decoder should provide a framework on how to integrate further adaptations on top of newer upstream.
• Enable with a one-liner the ability to load "haliumqsgcontext" as a render plugin for Qt. Integrates with mechanicd to ramp up scheduling of the Qt render thread. Could do more in the future.