pb: add void * for flush ctx to mapping functions

pb: add void * for flush ctx to mapping functions

If the buffer we are attempting to map is referenced by the unsubmitted
command stream for this context, we need to flush the command stream,
however to do that we need to be able to access the context at the lowest
level map function, currently we set the buffer in the toplevel map, but this
racy between context. (we probably have a lot more issues than that.)

I'll look into a proper solution as suggested by jrfonseca when I get some time.