[virglrenderer-devel] third submission of images

[Dave Airlie]

This is rebased onto master and is up to date with the feature work
and hopefully has the correct gles31 bits.

Since the last series I've pulled the membar and helper invocation
patches into this set, fixed some review comments from Tomeu in
patch 2, and fixed the protocol decode macros along the lines
of the outstanding ssbo patch.

Dave.