Merge changes from topic "sf-re-2"

* changes:
  libgui: add a helper for GLConsumer::getCurrentCrop
  libgui: use mat4 in GLConsumer
  libgui: remove unused GLConsumer methods
  libgui: add more IGBC wrappers to ConsumerBase
  libgui: tidy up CpuConsumer
  libgui: fix and clean up CpuConsumer::unlockBuffer
  libgui: fix a race in CpuConsumer::lockNextBuffer
  libgui: add helper to initialize CpuConsumer::LockedBuffer
  libgui: add helper to find CpuConsumer::AcquiredBuffer