* Generally this means removing ref out parameters and instead returning
values. In a couple of cases we will want to avoid copies in future
either by returning const references (e.g. to the pipeline state which
is immutable).
* At the same time, some pointless bool return values that were always
true and didn't indicate errors have been removed. They can be added
again if an error condition comes back.
* Some free functions still have out parameters as C linkage doesn't
allow returning user types by value.
* The C# UI still invokes into C wrappers for all the C++ classes, which
handle taking the return value and doing a copy into an out parameter
still for compatibility.
* This gives a little nicer syntax, a bit better type safety, and also
reflects better for SWIG bindings. Overall it's a minor change but
better.
* We don't update the C# UI at all, since it's soon to be removed and
not worth the effort/code churn.
* For now so we're ABI compatible with C#, all enums are uint32_t, but
that is an obvious optimisation in future to reduce struct packing.
* We avoid 'None' as an enum value, because it's a reserved word in
python so will cause problems generating bindings.
* If it was a blocking invoke, we'd release the processed semaphore then
the main thread could delete cmd before we check self delete (which
could be garbage and look like it's true) and delete it again.
* GUIInvoke helper changed to use QMetaObject::invokeMethod which works
on threads better.
* LambdaThread helper class now has a thread member, it doesn't derive
from thread (this seems to be recommended practice).
baldurk