Code review comment for lp:~alan-griffiths/mir/fix-1415321

I worked a bit on clarifying some of the nested/offscreen interfaces internally, but stopped short of splitting out a new .so (mostly so we don't have the maintenence burden of a new .so, and a new public distinction between "host" (android/mesa) and "guest" (nested/offscreen) "platforms", that only provide a display.). I think it is cleaner if mirclient only exports a c abi too, but I don't think this solution makes the dependency chain any more or less safe than the current status quo. I guess abstain for some more discussion on the MP.